diff --git a/.github/workflows/python-package.yml b/.github/workflows/python-package.yml
index 0ab3d142..d5b7a2d2 100644
--- a/.github/workflows/python-package.yml
+++ b/.github/workflows/python-package.yml
@@ -26,6 +26,22 @@ jobs:
           ruff check . --output-format=github
           ruff format --check .
 
+  type-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - name: Set up Python 3.13
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.13"
+          cache: "pip"
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install .[type-check]
+      - name: Type-check with mypy
+        run: mypy
+
   test:
     needs: lint
     runs-on: ${{ matrix.os }}
diff --git a/dataretrieval/nadp.py b/dataretrieval/nadp.py
index 3d1ee442..d6b26381 100644
--- a/dataretrieval/nadp.py
+++ b/dataretrieval/nadp.py
@@ -29,6 +29,8 @@
 
 """
 
+from __future__ import annotations
+
 import io
 import re
 import warnings
@@ -45,7 +47,7 @@
 )
 
 
-def _warn_deprecated():
+def _warn_deprecated() -> None:
     warnings.warn(_DEPRECATION_MESSAGE, DeprecationWarning, stacklevel=3)
 
 
@@ -74,19 +76,19 @@ def _warn_deprecated():
 class NADP_ZipFile(zipfile.ZipFile):
     """Extend zipfile.ZipFile for working on data from NADP"""
 
-    def tif_name(self):
+    def tif_name(self) -> str:
         """Get the name of the tif file in the zip file."""
         filenames = self.namelist()
         r = re.compile(".*tif$")
         tif_list = list(filter(r.match, filenames))
         return tif_list[0]
 
-    def tif(self):
+    def tif(self) -> bytes:
         """Read the tif file in the zip file."""
         return self.read(self.tif_name())
 
 
-def get_annual_MDN_map(measurement_type, year, path):
+def get_annual_MDN_map(measurement_type: str, year: str, path: str) -> str:
     """Download a MDN map from NDAP.
 
     This function looks for a zip file containing gridded information at:
@@ -135,7 +137,12 @@ def get_annual_MDN_map(measurement_type, year, path):
     return str(path)
 
 
-def get_annual_NTN_map(measurement_type, measurement=None, year=None, path="."):
+def get_annual_NTN_map(
+    measurement_type: str,
+    measurement: str | None = None,
+    year: str | None = None,
+    path: str = ".",
+) -> str:
     """Download a NTN map from NDAP.
 
     This function looks for a zip file containing gridded information at:
@@ -193,7 +200,7 @@ def get_annual_NTN_map(measurement_type, measurement=None, year=None, path="."):
     return str(path)
 
 
-def get_zip(url, filename):
+def get_zip(url: str, filename: str) -> NADP_ZipFile:
     """Gets a ZipFile at url and returns it
 
     Parameters
diff --git a/dataretrieval/nldi.py b/dataretrieval/nldi.py
index 8d61fcc2..a03aa1e6 100644
--- a/dataretrieval/nldi.py
+++ b/dataretrieval/nldi.py
@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from json import JSONDecodeError
-from typing import Literal
+from typing import Any, Literal, cast
 
 from dataretrieval.utils import query
 
@@ -16,13 +16,17 @@
 _VALID_NAVIGATION_MODES = ("UM", "DM", "UT", "DD")
 
 
-def _query_nldi(url, query_params, error_message):
+def _query_nldi(
+    url: str,
+    query_params: dict[str, str],
+    error_message: str,
+) -> dict[str, Any] | list[Any]:
     # A helper function to query the NLDI API
     response = query(url, payload=query_params)
     if response.status_code != 200:
         raise ValueError(f"{error_message}. Error reason: {response.reason_phrase}")
 
-    response_data = {}
+    response_data: dict[str, Any] | list[Any] = {}
     try:
         response_data = response.json()
     except JSONDecodeError:
@@ -32,7 +36,7 @@ def _query_nldi(url, query_params, error_message):
     return response_data
 
 
-def _features_to_gdf(feature_collection: dict) -> gpd.GeoDataFrame:
+def _features_to_gdf(feature_collection: dict[str, Any]) -> gpd.GeoDataFrame:
     """Build a GeoDataFrame from an NLDI FeatureCollection, tolerating empties.
 
     NLDI can legitimately return no features (e.g. a feature with nothing
@@ -56,7 +60,7 @@ def get_flowlines(
     stop_comid: int | None = None,
     trim_start: bool = False,
     as_json: bool = False,
-) -> gpd.GeoDataFrame | dict:
+) -> gpd.GeoDataFrame | dict[str, Any]:
     """Gets the flowlines for the specified navigation either by comid or feature
     source in WGS84 lat/long coordinates as GeoDataFrame containing a polyline geometry.
 
@@ -116,7 +120,7 @@ def get_flowlines(
     else:
         err_msg = f"Error getting flowlines for comid '{comid}'"
 
-    feature_collection = _query_nldi(url, query_params, err_msg)
+    feature_collection = cast("dict[str, Any]", _query_nldi(url, query_params, err_msg))
     if as_json:
         return feature_collection
     gdf = _features_to_gdf(feature_collection)
@@ -129,7 +133,7 @@ def get_basin(
     simplified: bool = True,
     split_catchment: bool = False,
     as_json: bool = False,
-) -> gpd.GeoDataFrame | dict:
+) -> gpd.GeoDataFrame | dict[str, Any]:
     """Gets the aggregated basin for the specified feature in WGS84 lat/lon
     as GeoDataFrame or as JSON conatining a polygon geometry.
 
@@ -162,14 +166,17 @@ def get_basin(
         raise ValueError("feature_id is required")
 
     url = f"{NLDI_API_BASE_URL}/{feature_source}/{feature_id}/basin"
-    simplified = str(simplified).lower()
-    split_catchment = str(split_catchment).lower()
-    query_params = {"simplified": simplified, "splitCatchment": split_catchment}
+    simplified_str = str(simplified).lower()
+    split_catchment_str = str(split_catchment).lower()
+    query_params = {
+        "simplified": simplified_str,
+        "splitCatchment": split_catchment_str,
+    }
     err_msg = (
         f"Error getting basin for feature source '{feature_source}' and "
         f"feature_id '{feature_id}'"
     )
-    feature_collection = _query_nldi(url, query_params, err_msg)
+    feature_collection = cast("dict[str, Any]", _query_nldi(url, query_params, err_msg))
     if as_json:
         return feature_collection
     gdf = _features_to_gdf(feature_collection)
@@ -187,7 +194,7 @@ def get_features(
     long: float | None = None,
     stop_comid: int | None = None,
     as_json: bool = False,
-) -> gpd.GeoDataFrame | dict:
+) -> gpd.GeoDataFrame | dict[str, Any]:
     """Gets all features found along the specified navigation either by
     comid or feature source as points in WGS84 lat/long coordinates - a GeoDataFrame
     containing a point geometry.
@@ -285,7 +292,7 @@ def get_features(
             query_params = {}
         err_msg = _features_err_msg(feature_source, feature_id, comid, data_source)
 
-    feature_collection = _query_nldi(url, query_params, err_msg)
+    feature_collection = cast("dict[str, Any]", _query_nldi(url, query_params, err_msg))
     if as_json:
         return feature_collection
     gdf = _features_to_gdf(feature_collection)
@@ -321,7 +328,7 @@ def get_features_by_data_source(data_source: str) -> gpd.GeoDataFrame:
     _validate_data_source(data_source)
     url = f"{NLDI_API_BASE_URL}/{data_source}"
     err_msg = f"Error getting features for data source '{data_source}'"
-    feature_collection = _query_nldi(url, {}, err_msg)
+    feature_collection = cast("dict[str, Any]", _query_nldi(url, {}, err_msg))
     gdf = _features_to_gdf(feature_collection)
     return gdf
 
@@ -336,7 +343,7 @@ def search(
     lat: float | None = None,
     long: float | None = None,
     distance: int = 50,
-) -> dict:
+) -> dict[str, Any]:
     """Searches for the specified feature in NLDI and returns the results
     as a dictionary.
 
@@ -408,7 +415,7 @@ def search(
     if (lat is None) != (long is None):
         raise ValueError("Both lat and long are required")
 
-    find = find.lower()
+    find = cast(Literal["basin", "flowlines", "features"], find.lower())
     if find not in ("basin", "flowlines", "features"):
         raise ValueError(
             f"Invalid value for find: {find} - allowed values are:"
@@ -428,6 +435,10 @@ def search(
         return get_features(lat=lat, long=long, as_json=True)
 
     if find == "basin":
+        if feature_source is None or feature_id is None:
+            raise ValueError(
+                "feature_source and feature_id are required to find a basin"
+            )
         return get_basin(
             feature_source=feature_source, feature_id=feature_id, as_json=True
         )
@@ -458,7 +469,7 @@ def search(
     )
 
 
-def _validate_data_source(data_source: str):
+def _validate_data_source(data_source: str) -> None:
     # A helper function to validate user specified data source/feature source
 
     global _AVAILABLE_DATA_SOURCES
@@ -487,7 +498,12 @@ def _validate_data_source(data_source: str):
         raise ValueError(err_msg)
 
 
-def _features_err_msg(feature_source, feature_id, comid, data_source) -> str:
+def _features_err_msg(
+    feature_source: str | None,
+    feature_id: str | None,
+    comid: int | None,
+    data_source: str | None,
+) -> str:
     if feature_source is not None:
         return (
             f"Error getting features for feature source '{feature_source}'"
@@ -512,7 +528,7 @@ def _validate_navigation_mode(navigation_mode: str | None) -> str:
 
 def _validate_feature_source_comid(
     feature_source: str | None, feature_id: str | None, comid: int | None
-):
+) -> None:
     if feature_source is not None and feature_id is None:
         raise ValueError("feature_id is required if feature_source is provided")
     if feature_id is not None and feature_source is None:
diff --git a/dataretrieval/nwis.py b/dataretrieval/nwis.py
index 1372caa7..fafd0a08 100644
--- a/dataretrieval/nwis.py
+++ b/dataretrieval/nwis.py
@@ -9,7 +9,9 @@
 import functools
 import threading
 import warnings
+from collections.abc import Callable
 from json import JSONDecodeError
+from typing import Any, NoReturn, TypeVar, cast
 
 import httpx
 import pandas as pd
@@ -24,6 +26,8 @@
 except ImportError:
     gpd = None
 
+F = TypeVar("F", bound=Callable[..., Any])
+
 WATERDATA_BASE_URL = "https://nwis.waterdata.usgs.gov/"
 WATERDATA_URL = WATERDATA_BASE_URL + "nwis/"
 WATERSERVICE_URL = "https://waterservices.usgs.gov/nwis/"
@@ -75,7 +79,7 @@ def _warn_deprecated(func_name: str) -> None:
     )
 
 
-def _deprecated(func):
+def _deprecated(func: F) -> F:
     """Mark an nwis function as deprecated.
 
     Wrappers like ``get_record`` -> ``get_iv`` -> ``query_waterservices`` would
@@ -89,7 +93,7 @@ def _deprecated(func):
         )
 
     @functools.wraps(func)
-    def wrapper(*args, **kwargs):
+    def wrapper(*args: Any, **kwargs: Any) -> Any:
         if getattr(_deprecation_state, "active", False):
             return func(*args, **kwargs)
         _deprecation_state.active = True
@@ -99,7 +103,7 @@ def wrapper(*args, **kwargs):
         finally:
             _deprecation_state.active = False
 
-    return wrapper
+    return cast(F, wrapper)
 
 
 def _parse_json_or_raise(response: httpx.Response) -> pd.DataFrame:
@@ -123,7 +127,7 @@ def _parse_json_or_raise(response: httpx.Response) -> pd.DataFrame:
 
 
 def format_response(
-    df: pd.DataFrame, service: str | None = None, **kwargs
+    df: pd.DataFrame, service: str | None = None, **kwargs: Any
 ) -> pd.DataFrame:
     """Setup index for response from query.
 
@@ -197,14 +201,14 @@ def preformat_peaks_response(df: pd.DataFrame) -> pd.DataFrame:
     return df
 
 
-def get_qwdata(**kwargs):
+def get_qwdata(**kwargs: Any) -> NoReturn:
     """Defunct: use ``waterdata.get_samples()``."""
     raise NameError(
         "`nwis.get_qwdata` has been replaced with `waterdata.get_samples()`."
     )
 
 
-def get_discharge_measurements(**kwargs):
+def get_discharge_measurements(**kwargs: Any) -> NoReturn:
     """Defunct: use ``waterdata.get_field_measurements()``."""
     raise NameError(
         "`nwis.get_discharge_measurements` has been replaced "
@@ -219,8 +223,8 @@ def get_discharge_peaks(
     end: str | None = None,
     multi_index: bool = True,
     ssl_check: bool = True,
-    **kwargs,
-) -> tuple[pd.DataFrame, BaseMetadata]:
+    **kwargs: Any,
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Get discharge peaks from the waterdata service.
 
@@ -285,7 +289,7 @@ def get_discharge_peaks(
     )
 
 
-def get_gwlevels(**kwargs):
+def get_gwlevels(**kwargs: Any) -> NoReturn:
     """Defunct: use ``waterdata.get_continuous()``, ``waterdata.get_daily()``,
     or ``waterdata.get_field_measurements()``."""
     raise NameError(
@@ -298,8 +302,8 @@ def get_gwlevels(**kwargs):
 
 @_deprecated
 def get_stats(
-    sites: list[str] | str | None = None, ssl_check: bool = True, **kwargs
-) -> tuple[pd.DataFrame, BaseMetadata]:
+    sites: list[str] | str | None = None, ssl_check: bool = True, **kwargs: Any
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Queries water services statistics information.
 
@@ -359,7 +363,9 @@ def get_stats(
 
 
 @_deprecated
-def query_waterdata(service: str, ssl_check: bool = True, **kwargs) -> httpx.Response:
+def query_waterdata(
+    service: str, ssl_check: bool = True, **kwargs: Any
+) -> httpx.Response:
     """
     Queries waterdata.
 
@@ -404,7 +410,7 @@ def query_waterdata(service: str, ssl_check: bool = True, **kwargs) -> httpx.Res
 
 @_deprecated
 def query_waterservices(
-    service: str, ssl_check: bool = True, **kwargs
+    service: str, ssl_check: bool = True, **kwargs: Any
 ) -> httpx.Response:
     """
     Queries waterservices.usgs.gov
@@ -473,8 +479,8 @@ def get_dv(
     end: str | None = None,
     multi_index: bool = True,
     ssl_check: bool = True,
-    **kwargs,
-) -> tuple[pd.DataFrame, BaseMetadata]:
+    **kwargs: Any,
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Get daily values data from NWIS and return it as a ``pandas.DataFrame``.
 
@@ -539,7 +545,9 @@ def get_dv(
 
 
 @_deprecated
-def get_info(ssl_check: bool = True, **kwargs) -> tuple[pd.DataFrame, BaseMetadata]:
+def get_info(
+    ssl_check: bool = True, **kwargs: Any
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Get site description information from NWIS.
 
@@ -661,8 +669,8 @@ def get_iv(
     end: str | None = None,
     multi_index: bool = True,
     ssl_check: bool = True,
-    **kwargs,
-) -> tuple[pd.DataFrame, BaseMetadata]:
+    **kwargs: Any,
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """Get instantaneous values data from NWIS and return it as a DataFrame.
 
     .. note::
@@ -725,7 +733,7 @@ def get_iv(
     return format_response(df, **kwargs), NWIS_Metadata(response, **kwargs)
 
 
-def get_pmcodes(**kwargs):
+def get_pmcodes(**kwargs: Any) -> NoReturn:
     """Defunct: use ``get_reference_table(collection='parameter-codes')``."""
     raise NameError(
         "`nwis.get_pmcodes` has been replaced "
@@ -733,7 +741,7 @@ def get_pmcodes(**kwargs):
     )
 
 
-def get_water_use(**kwargs):
+def get_water_use(**kwargs: Any) -> NoReturn:
     """Defunct: no current replacement."""
     raise NameError("`nwis.get_water_use` is defunct.")
 
@@ -743,8 +751,8 @@ def get_ratings(
     site: str | None = None,
     file_type: str = "base",
     ssl_check: bool = True,
-    **kwargs,
-) -> tuple[pd.DataFrame, BaseMetadata]:
+    **kwargs: Any,
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Rating table for an active USGS streamgage retrieval.
 
@@ -797,7 +805,9 @@ def get_ratings(
 
 
 @_deprecated
-def what_sites(ssl_check: bool = True, **kwargs) -> tuple[pd.DataFrame, BaseMetadata]:
+def what_sites(
+    ssl_check: bool = True, **kwargs: Any
+) -> tuple[pd.DataFrame, NWIS_Metadata]:
     """
     Search NWIS for sites within a region with specific data.
 
@@ -847,7 +857,7 @@ def get_record(
     state: str | None = None,
     service: str = "iv",
     ssl_check: bool = True,
-    **kwargs,
+    **kwargs: Any,
 ) -> pd.DataFrame:
     """
     Get data from NWIS and return it as a ``pandas.DataFrame``.
@@ -985,7 +995,10 @@ def get_record(
         return df
 
     elif service == "ratings":
-        df, _ = get_ratings(site=sites, ssl_check=ssl_check, **kwargs)
+        # the ratings service is single-site; get_ratings takes a scalar site
+        df, _ = get_ratings(
+            site=cast("str | None", sites), ssl_check=ssl_check, **kwargs
+        )
         return df
 
     elif service == "stat":
@@ -996,7 +1009,7 @@ def get_record(
         raise TypeError(f"{service} service not yet implemented")
 
 
-def _read_json(json):
+def _read_json(json: dict[str, Any]) -> pd.DataFrame:
     """
     Reads a NWIS Water Services formatted JSON into a ``pandas.DataFrame``.
 
@@ -1092,7 +1105,7 @@ def _read_json(json):
     return merged_df
 
 
-def _read_rdb(rdb):
+def _read_rdb(rdb: str) -> pd.DataFrame:
     """Parse an NWIS RDB response and apply NWIS-specific post-processing.
 
     Thin wrapper around :func:`dataretrieval.rdb.read_rdb` that adds the
@@ -1102,7 +1115,7 @@ def _read_rdb(rdb):
     return format_response(read_rdb(rdb, dtypes=_NWIS_RDB_DTYPES))
 
 
-def _check_sites_value_types(sites):
+def _check_sites_value_types(sites: list[str] | str | None) -> None:
     if sites and not isinstance(sites, list) and not isinstance(sites, str):
         raise TypeError("sites must be a string or a list of strings")
 
@@ -1128,7 +1141,7 @@ class NWIS_Metadata(BaseMetadata):
 
     """
 
-    def __init__(self, response, **parameters) -> None:
+    def __init__(self, response: httpx.Response, **parameters: Any) -> None:
         """Generates a standard set of metadata informed by the response with specific
         metadata for NWIS data.
 
diff --git a/dataretrieval/samples.py b/dataretrieval/samples.py
index 2259969c..025fa76e 100644
--- a/dataretrieval/samples.py
+++ b/dataretrieval/samples.py
@@ -7,9 +7,15 @@
 from __future__ import annotations
 
 import warnings
+from typing import TYPE_CHECKING, Any
 
+if TYPE_CHECKING:
+    import pandas as pd
 
-def get_usgs_samples(**kwargs):
+    from dataretrieval.utils import BaseMetadata
+
+
+def get_usgs_samples(**kwargs: Any) -> tuple[pd.DataFrame, BaseMetadata]:
     """Deprecated: use ``waterdata.get_samples()`` instead.
 
     All keyword arguments are forwarded directly to
diff --git a/dataretrieval/streamstats.py b/dataretrieval/streamstats.py
index 6737d54c..039f292b 100644
--- a/dataretrieval/streamstats.py
+++ b/dataretrieval/streamstats.py
@@ -5,14 +5,17 @@
 
 """
 
+from __future__ import annotations
+
 import json
+from typing import Any, cast
 
 import httpx
 
 from dataretrieval.utils import HTTPX_DEFAULTS
 
 
-def download_workspace(workspaceID, format=""):
+def download_workspace(workspaceID: str, format: str = "") -> httpx.Response:
     """Function to download streamstats workspace.
 
     Parameters
@@ -46,7 +49,7 @@ def download_workspace(workspaceID, format=""):
     # return
 
 
-def get_sample_watershed():
+def get_sample_watershed() -> Watershed:
     """Sample function to get a watershed object for a location in NY.
 
     Makes the function call :obj:`dataretrieval.streamstats.get_watershed`
@@ -60,20 +63,23 @@ def get_sample_watershed():
         from the streamstats JSON object.
 
     """
-    return get_watershed("NY", -74.524, 43.939, format="object")
+    return cast(
+        "Watershed",
+        get_watershed("NY", -74.524, 43.939, format="object"),
+    )
 
 
 def get_watershed(
-    rcode,
-    xlocation,
-    ylocation,
-    crs=4326,
-    includeparameters=True,
-    includeflowtypes=False,
-    includefeatures=True,
-    simplify=True,
-    format="geojson",
-):
+    rcode: str,
+    xlocation: float,
+    ylocation: float,
+    crs: int | str = 4326,
+    includeparameters: bool = True,
+    includeflowtypes: bool = False,
+    includefeatures: bool = True,
+    simplify: bool = True,
+    format: str = "geojson",
+) -> httpx.Response | Watershed:
     """Get watershed object based on location
 
     **Streamstats documentation:**
@@ -115,7 +121,7 @@ def get_watershed(
         from the streamstats JSON object.
 
     """
-    payload = {
+    payload: dict[str, str | int | float | bool] = {
         "rcode": rcode,
         "xlocation": xlocation,
         "ylocation": ylocation,
@@ -170,14 +176,17 @@ class Watershed:
         :obj:`dataretrieval.streamstats.download_workspace`.
     """
 
-    def __init__(self, rcode, xlocation, ylocation):
+    def __init__(self, rcode: str, xlocation: float, ylocation: float) -> None:
         """Delineate the watershed at ``(xlocation, ylocation)`` and
         parse the response onto this instance."""
-        response = get_watershed(rcode, xlocation, ylocation, format="geojson")
+        response = cast(
+            httpx.Response,
+            get_watershed(rcode, xlocation, ylocation, format="geojson"),
+        )
         self._populate(json.loads(response.text))
 
     @classmethod
-    def from_streamstats_json(cls, streamstats_json) -> "Watershed":
+    def from_streamstats_json(cls, streamstats_json: dict[str, Any]) -> Watershed:
         """Create a :class:`Watershed` from an already-parsed StreamStats
         JSON payload, without issuing a new request.
 
@@ -190,7 +199,7 @@ class state.
         self._populate(streamstats_json)
         return self
 
-    def _populate(self, streamstats_json) -> None:
+    def _populate(self, streamstats_json: dict[str, Any]) -> None:
         """Extract watershed fields from a StreamStats JSON payload onto
         this instance."""
         self.watershed_point = streamstats_json["featurecollection"][0]["feature"]
diff --git a/dataretrieval/utils.py b/dataretrieval/utils.py
index 7bb03a69..f9766ee6 100644
--- a/dataretrieval/utils.py
+++ b/dataretrieval/utils.py
@@ -2,8 +2,11 @@
 Useful utilities for data munging.
 """
 
+from __future__ import annotations
+
 import warnings
 from collections.abc import Iterable
+from typing import Any
 
 import httpx
 import pandas as pd
@@ -11,13 +14,16 @@
 import dataretrieval
 from dataretrieval.codes import tz
 
-HTTPX_DEFAULTS = {
+# Typed as ``dict[str, Any]`` (not the inferred ``dict[str, object]``) so that
+# splatting it as ``**HTTPX_DEFAULTS`` into ``httpx.get`` / ``httpx.AsyncClient``
+# type-checks: the values are a heterogeneous bag of httpx keyword arguments.
+HTTPX_DEFAULTS: dict[str, Any] = {
     "follow_redirects": True,
     "timeout": httpx.Timeout(60.0, connect=10.0),
 }
 
 
-def to_str(listlike, delimiter=","):
+def to_str(listlike: object, delimiter: str = ",") -> str | None:
     """Translates list-like objects into strings.
 
     Parameters
@@ -54,7 +60,9 @@ def to_str(listlike, delimiter=","):
     return None
 
 
-def format_datetime(df, date_field, time_field, tz_field):
+def format_datetime(
+    df: pd.DataFrame, date_field: str, time_field: str, tz_field: str
+) -> pd.DataFrame:
     """Creates a datetime field from separate date, time, and
     time zone fields.
 
@@ -190,6 +198,7 @@ def _attach_datetime_columns(df: pd.DataFrame) -> pd.DataFrame:
         # Concat in one shot — per-column assignment on a wide CSV-derived
         # frame triggers pandas' fragmentation PerformanceWarning.
         df = pd.concat([df, pd.DataFrame(new_columns, index=df.index)], axis=1)
+    sort_key: str | None
     if "Activity_StartDateTime" in df.columns:
         sort_key = "Activity_StartDateTime"
     elif "ActivityStartDateTime" in df.columns:
@@ -215,7 +224,7 @@ class BaseMetadata:
 
     """
 
-    def __init__(self, response) -> None:
+    def __init__(self, response: httpx.Response) -> None:
         """Generates a standard set of metadata informed by the response.
 
         Parameters
@@ -234,7 +243,7 @@ def __init__(self, response) -> None:
         self.url = str(response.url)
         self.query_time = response.elapsed
         self.header = response.headers
-        self.comment = None
+        self.comment: str | None = None
 
         # # not sure what statistic_info is
         # self.statistic_info = None
@@ -244,13 +253,13 @@ def __init__(self, response) -> None:
 
     # These properties are to be set by `nwis` or `wqp`-specific metadata classes.
     @property
-    def site_info(self):
+    def site_info(self) -> Any:
         raise NotImplementedError(
             "site_info must be implemented by utils.BaseMetadata children"
         )
 
     @property
-    def variable_info(self):
+    def variable_info(self) -> Any:
         raise NotImplementedError(
             "variable_info must be implemented by utils.BaseMetadata children"
         )
@@ -278,7 +287,12 @@ def _url_too_long_error(detail: str) -> ValueError:
     )
 
 
-def query(url, payload, delimiter=",", ssl_check=True):
+def query(
+    url: str,
+    payload: dict[str, Any],
+    delimiter: str = ",",
+    ssl_check: bool = True,
+) -> httpx.Response:
     """Send a query.
 
     Wrapper for httpx.get that handles errors, converts listed
@@ -347,10 +361,10 @@ def query(url, payload, delimiter=",", ssl_check=True):
 class NoSitesError(Exception):
     """Custom error class used when selection criteria returns no sites/data."""
 
-    def __init__(self, url):
+    def __init__(self, url: httpx.URL) -> None:
         self.url = url
 
-    def __str__(self):
+    def __str__(self) -> str:
         return (
             "No sites/data found using the selection criteria specified in "
             f"url: {self.url}"
diff --git a/dataretrieval/waterdata/api.py b/dataretrieval/waterdata/api.py
index 1b609324..3144bf80 100644
--- a/dataretrieval/waterdata/api.py
+++ b/dataretrieval/waterdata/api.py
@@ -10,7 +10,7 @@
 import logging
 from collections.abc import Iterable
 from io import StringIO
-from typing import get_args
+from typing import Any, get_args
 from urllib.parse import quote
 
 import httpx
@@ -2018,7 +2018,7 @@ def get_peaks(
 def get_reference_table(
     collection: str,
     limit: int | None = None,
-    query: dict | None = None,
+    query: dict[str, Any] | None = None,
     max_rows: int | None = None,
 ) -> tuple[pd.DataFrame, BaseMetadata]:
     """Get metadata reference tables for the USGS Water Data API.
@@ -2140,7 +2140,7 @@ def get_codes(code_service: CODE_SERVICES) -> tuple[pd.DataFrame, BaseMetadata]:
 
 
 def _get_samples_csv(
-    url: str, params: dict, ssl_check: bool
+    url: str, params: dict[str, Any], ssl_check: bool
 ) -> tuple[pd.DataFrame, httpx.Response]:
     """Issue a Samples CSV request and parse the body into a DataFrame.
 
@@ -2852,7 +2852,7 @@ def get_channel(
 
 def get_cql(
     service: WATERDATA_SERVICES,
-    cql: str | dict,
+    cql: str | dict[str, Any],
     *,
     properties: str | Iterable[str] | None = None,
     bbox: list[float] | None = None,
diff --git a/dataretrieval/waterdata/chunking.py b/dataretrieval/waterdata/chunking.py
index ab079070..1f49ed97 100644
--- a/dataretrieval/waterdata/chunking.py
+++ b/dataretrieval/waterdata/chunking.py
@@ -59,7 +59,7 @@
 from contextvars import ContextVar
 from dataclasses import dataclass
 from datetime import timedelta
-from typing import Any, ClassVar
+from typing import Any, ClassVar, cast
 from urllib.parse import quote_plus
 
 import httpx
@@ -681,7 +681,7 @@ def _set_response_url(response: httpx.Response, url: str | httpx.URL) -> None:
     same ``.request``.
     """
     try:
-        response.url = url  # type: ignore[misc]
+        response.url = url  # type: ignore[misc, assignment]
     except AttributeError:
         target = httpx.URL(str(url))
         try:
@@ -800,7 +800,7 @@ def _extract_axes(args: dict[str, Any]) -> list[_Axis]:
             axes.append(_Axis(arg_key=key, atoms=tuple(value), joiner=_LIST_SEP))
 
     filter_expr = args.get("filter")
-    if _is_chunkable(filter_expr, args.get("filter_lang")):
+    if filter_expr is not None and _is_chunkable(filter_expr, args.get("filter_lang")):
         _check_numeric_filter_pitfall(filter_expr)
         clauses = _split_top_level_or(filter_expr)
         if len(clauses) >= 2:
@@ -1560,7 +1560,12 @@ def resume(self) -> tuple[pd.DataFrame, Any]:
         """
         concurrency = _read_concurrency_env()
         with start_blocking_portal() as portal:
-            return portal.call(functools.partial(self._run, concurrency))
+            # ``portal.call`` returns ``Any`` because ``functools.partial``
+            # erases ``_run``'s return type; restore the declared tuple.
+            return cast(
+                "tuple[pd.DataFrame, Any]",
+                portal.call(functools.partial(self._run, concurrency)),
+            )
 
     async def _run(self, max_concurrent: int | None) -> tuple[pd.DataFrame, Any]:
         """
diff --git a/dataretrieval/waterdata/nearest.py b/dataretrieval/waterdata/nearest.py
index 12aad61c..39a80332 100644
--- a/dataretrieval/waterdata/nearest.py
+++ b/dataretrieval/waterdata/nearest.py
@@ -6,7 +6,7 @@
 from __future__ import annotations
 
 from collections.abc import Iterable
-from typing import Literal, get_args
+from typing import Any, Literal, get_args
 
 import pandas as pd
 
@@ -18,13 +18,13 @@
 
 
 def get_nearest_continuous(
-    targets,
+    targets: Iterable[Any],
     monitoring_location_id: str | Iterable[str] | None = None,
     parameter_code: str | Iterable[str] | None = None,
     *,
     window: str | pd.Timedelta = "PT7M30S",
     on_tie: OnTie = "first",
-    **kwargs,
+    **kwargs: Any,
 ) -> tuple[pd.DataFrame, BaseMetadata]:
     """For each target timestamp, return the nearest continuous observation.
 
@@ -138,13 +138,13 @@ def get_nearest_continuous(
         ... )
     """
     _check_nearest_kwargs(kwargs, on_tie)
-    targets = pd.DatetimeIndex(pd.to_datetime(targets, utc=True))
+    target_index = pd.DatetimeIndex(pd.to_datetime(targets, utc=True))
     window_td = pd.Timedelta(window)
 
-    if len(targets) == 0:
+    if len(target_index) == 0:
         raise ValueError("targets must contain at least one timestamp")
 
-    filter_expr = _build_window_or_filter(targets, window_td)
+    filter_expr = _build_window_or_filter(target_index, window_td)
     df, md = get_continuous(
         monitoring_location_id=monitoring_location_id,
         parameter_code=parameter_code,
@@ -165,7 +165,7 @@ def get_nearest_continuous(
     selected = [
         row
         for _, site_df in site_groups
-        for target in targets
+        for target in target_index
         if (row := _pick_nearest_row(site_df, target, window_td, on_tie)) is not None
     ]
     if not selected:
@@ -173,7 +173,7 @@ def get_nearest_continuous(
     return pd.DataFrame(selected).reset_index(drop=True), md
 
 
-def _check_nearest_kwargs(kwargs: dict, on_tie: OnTie) -> None:
+def _check_nearest_kwargs(kwargs: dict[str, Any], on_tie: OnTie) -> None:
     """Reject kwargs the helper owns; validate ``on_tie``."""
     for forbidden in ("time", "filter", "filter_lang"):
         if forbidden in kwargs:
diff --git a/dataretrieval/waterdata/ratings.py b/dataretrieval/waterdata/ratings.py
index ed242612..c0f870c1 100644
--- a/dataretrieval/waterdata/ratings.py
+++ b/dataretrieval/waterdata/ratings.py
@@ -246,15 +246,18 @@ def _search(
     STAC ``next`` link is followed until exhausted so a result set larger than
     one page isn't silently truncated.
     """
-    params: dict[str, Any] | None = {"limit": min(limit, 10000)}
+    query_params: dict[str, Any] = {"limit": min(limit, 10000)}
     if filter_str is not None:
-        params["filter"] = filter_str
+        query_params["filter"] = filter_str
     if time_str is not None:
-        params["datetime"] = time_str
+        query_params["datetime"] = time_str
     if bbox is not None:
-        params["bbox"] = ",".join(map(str, bbox))
+        query_params["bbox"] = ",".join(map(str, bbox))
 
     url: str | None = f"{STAC_URL}/search"
+    # ``params`` is sent only on the first request; each STAC ``next`` link
+    # already carries the query, so it is reset to None inside the loop.
+    params: dict[str, Any] | None = query_params
     features: list[dict[str, Any]] = []
     while url is not None:
         response = httpx.get(
diff --git a/dataretrieval/waterdata/utils.py b/dataretrieval/waterdata/utils.py
index ad1b3afd..5581d086 100644
--- a/dataretrieval/waterdata/utils.py
+++ b/dataretrieval/waterdata/utils.py
@@ -14,11 +14,12 @@
     Iterable,
     Iterator,
     Mapping,
+    Sequence,
 )
 from contextlib import asynccontextmanager, contextmanager
 from contextvars import ContextVar
 from datetime import datetime, timedelta
-from typing import Any, TypeVar, get_args
+from typing import Any, TypeVar, cast, get_args
 from zoneinfo import ZoneInfo
 
 import httpx
@@ -97,7 +98,7 @@
 }
 
 
-def _switch_arg_id(ls: dict[str, Any], id_name: str, service: str):
+def _switch_arg_id(ls: dict[str, Any], id_name: str, service: str) -> dict[str, Any]:
     """
     Switch argument id from its package-specific identifier to the standardized "id" key
     that the API recognizes.
@@ -142,7 +143,9 @@ def _switch_arg_id(ls: dict[str, Any], id_name: str, service: str):
     return ls
 
 
-def _switch_properties_id(properties: list[str] | None, id_name: str, service: str):
+def _switch_properties_id(
+    properties: list[str] | None, id_name: str, service: str
+) -> list[str]:
     """
     Switch properties id from its package-specific identifier to the
     standardized "id" key that the API recognizes.
@@ -233,7 +236,7 @@ def _parse_datetime(value: str) -> datetime | None:
     return None
 
 
-def _format_one(dt, *, date: bool) -> str | None:
+def _format_one(dt: str | None, *, date: bool) -> str | None:
     """Format a single datetime element for inclusion in the API time arg."""
     if pd.isna(dt) or dt == "" or dt is None:
         return ".."
@@ -251,7 +254,7 @@ def _format_one(dt, *, date: bool) -> str | None:
 
 
 def _format_api_dates(
-    datetime_input: str | list[str | None] | None, date: bool = False
+    datetime_input: str | Sequence[str | None] | None, date: bool = False
 ) -> str | None:
     """
     Formats date or datetime input(s) for use with an API.
@@ -330,11 +333,13 @@ def _format_api_dates(
         if _DURATION_RE.match(single) or "/" in single:
             return single
 
-    # Half-bounded ranges: NA endpoints render as ".."; any unparseable non-NA
     # element invalidates the range.
-    formatted = [_format_one(dt, date=date) for dt in datetime_input]
-    if any(f is None for f in formatted):
-        return None
+    formatted: list[str] = []
+    for dt in datetime_input:
+        one = _format_one(dt, date=date)
+        if one is None:
+            return None
+        formatted.append(one)
     return "/".join(formatted)
 
 
@@ -371,7 +376,7 @@ def _cql2_param(args: dict[str, Any]) -> str:
     return json.dumps(query, separators=(",", ":"))
 
 
-def _default_headers():
+def _default_headers() -> dict[str, str]:
     """
     Generate default HTTP headers for API requests.
 
@@ -394,7 +399,9 @@ def _default_headers():
     return headers
 
 
-def _check_ogc_requests(endpoint: str = "daily", req_type: str = "queryables"):
+def _check_ogc_requests(
+    endpoint: str = "daily", req_type: str = "queryables"
+) -> dict[str, Any]:
     """
     Sends an HTTP GET request to the specified OGC endpoint and request type,
     returning the JSON response.
@@ -426,10 +433,12 @@ def _check_ogc_requests(endpoint: str = "daily", req_type: str = "queryables"):
     url = f"{OGC_API_URL}/collections/{endpoint}/{req_type}"
     resp = httpx.get(url, headers=_default_headers(), **HTTPX_DEFAULTS)
     _raise_for_non_200(resp)
-    return resp.json()
+    # ``Response.json`` is typed ``Any``; the OGC queryables/schema endpoints
+    # return a JSON object, and callers index it as a dict.
+    return cast("dict[str, Any]", resp.json())
 
 
-def _error_body(resp: httpx.Response):
+def _error_body(resp: httpx.Response) -> str:
     """
     Build an informative error message from an HTTP response.
 
@@ -626,7 +635,7 @@ def _construct_api_requests(
     bbox: list[float] | None = None,
     limit: int | None = None,
     skip_geometry: bool = False,
-    **kwargs,
+    **kwargs: Any,
 ) -> httpx.Request:
     """
     Constructs an HTTP request object for the specified water data API service.
@@ -823,6 +832,8 @@ def _next_req_url(
         # body might supply. Guarded against mock-shaped ``resp.url``
         # attributes (tests sometimes set strings or ``MagicMock``)
         # by falling open when host extraction isn't reliable.
+        next_host: str | None
+        cur_host: str | None
         try:
             next_host = httpx.URL(href).host
             resp_url = (
@@ -838,7 +849,10 @@ def _next_req_url(
                 f"Refusing to follow cross-host next-page URL: "
                 f"{next_host} != {cur_host}"
             )
-        return href
+        # ``href`` comes from the JSON ``links`` array (typed ``Any``); the
+        # ``not href`` guard above already excluded empty/None, and it is a
+        # URL string (passed to ``httpx.URL`` above).
+        return cast("str", href)
     return None
 
 
@@ -1915,11 +1929,10 @@ def _as_str_list(
     ``",".join(...)`` doesn't iterate it character-by-character — and
     materializes any other iterable via :func:`_normalize_str_iterable`.
     """
-    return (
-        [value]
-        if isinstance(value, str)
-        else _normalize_str_iterable(value, param_name)
-    )
+    normalized = _normalize_str_iterable(value, param_name)
+    if isinstance(normalized, str):
+        return [normalized]
+    return normalized
 
 
 def _check_monitoring_location_id(
diff --git a/dataretrieval/wqp.py b/dataretrieval/wqp.py
index ff01c46a..1ca0098a 100644
--- a/dataretrieval/wqp.py
+++ b/dataretrieval/wqp.py
@@ -13,13 +13,14 @@
 
 import warnings
 from io import StringIO
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Any
 
 import pandas as pd
 
 from .utils import BaseMetadata, _attach_datetime_columns, query
 
 if TYPE_CHECKING:
+    import httpx
     from pandas import DataFrame
 
 
@@ -67,9 +68,9 @@ def _read_wqp_csv(text: str) -> DataFrame:
 
 
 def get_results(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Query the WQP for results.
 
@@ -185,9 +186,9 @@ def get_results(
 
 
 def what_sites(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for sites within a region with specific data.
 
@@ -240,9 +241,9 @@ def what_sites(
 
 
 def what_organizations(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for organizations within a region with specific data.
 
@@ -290,7 +291,11 @@ def what_organizations(
     return df, WQP_Metadata(response, **kwargs)
 
 
-def what_projects(ssl_check=True, legacy=True, **kwargs):
+def what_projects(
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
+) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for projects within a region with specific data.
 
     Any WQP API parameter can be passed as a keyword argument to this function.
@@ -338,9 +343,9 @@ def what_projects(ssl_check=True, legacy=True, **kwargs):
 
 
 def what_activities(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for activities within a region with specific data.
 
@@ -402,9 +407,9 @@ def what_activities(
 
 
 def what_detection_limits(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for result detection limits within a region with specific
     data.
@@ -460,9 +465,9 @@ def what_detection_limits(
 
 
 def what_habitat_metrics(
-    ssl_check=True,
-    legacy=True,
-    **kwargs,
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
 ) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for habitat metrics within a region with specific data.
 
@@ -510,7 +515,11 @@ def what_habitat_metrics(
     return df, WQP_Metadata(response, **kwargs)
 
 
-def what_project_weights(ssl_check=True, legacy=True, **kwargs):
+def what_project_weights(
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
+) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for project weights within a region with specific data.
 
     Any WQP API parameter can be passed as a keyword argument to this function.
@@ -562,7 +571,11 @@ def what_project_weights(ssl_check=True, legacy=True, **kwargs):
     return df, WQP_Metadata(response, **kwargs)
 
 
-def what_activity_metrics(ssl_check=True, legacy=True, **kwargs):
+def what_activity_metrics(
+    ssl_check: bool = True,
+    legacy: bool = True,
+    **kwargs: Any,
+) -> tuple[DataFrame, WQP_Metadata]:
     """Search WQP for activity metrics within a region with specific data.
 
     Any WQP API parameter can be passed as a keyword argument to this function.
@@ -614,7 +627,7 @@ def what_activity_metrics(ssl_check=True, legacy=True, **kwargs):
     return df, WQP_Metadata(response, **kwargs)
 
 
-def wqp_url(service):
+def wqp_url(service: str) -> str:
     """Construct the WQP URL for a given service."""
 
     base_url = "https://www.waterqualitydata.us/data/"
@@ -628,7 +641,7 @@ def wqp_url(service):
     return f"{base_url}{service}/Search?"
 
 
-def wqx3_url(service):
+def wqx3_url(service: str) -> str:
     """Construct the WQP URL for a given WQX 3.0 service."""
 
     base_url = "https://www.waterqualitydata.us/wqx3/"
@@ -659,7 +672,7 @@ class WQP_Metadata(BaseMetadata):
         Site information (via ``what_sites``) if the query included a ``siteid``.
     """
 
-    def __init__(self, response, **parameters) -> None:
+    def __init__(self, response: httpx.Response, **parameters: Any) -> None:
         """Generates a standard set of metadata informed by the response with specific
         metadata for WQP data.
 
@@ -703,7 +716,7 @@ def site_info(self) -> tuple[DataFrame, WQP_Metadata] | None:
         return what_sites(siteid=siteid)
 
 
-def _check_kwargs(kwargs):
+def _check_kwargs(kwargs: dict[str, Any]) -> dict[str, Any]:
     """Private function to check kwargs for unsupported parameters."""
     mimetype = kwargs.get("mimeType")
     if mimetype == "geojson":
@@ -716,7 +729,7 @@ def _check_kwargs(kwargs):
     return kwargs
 
 
-def _warn_wqx3_use():
+def _warn_wqx3_use() -> None:
     message = (
         "Support for the WQX3.0 profiles is experimental. "
         "Queries may be slow or fail intermittently."
@@ -724,7 +737,7 @@ def _warn_wqx3_use():
     warnings.warn(message, UserWarning, stacklevel=2)
 
 
-def _warn_legacy_use():
+def _warn_legacy_use() -> None:
     message = (
         "This function call will return the legacy WQX format, "
         "which means USGS data have not been updated since March 2024. "
@@ -735,7 +748,7 @@ def _warn_legacy_use():
     warnings.warn(message, DeprecationWarning, stacklevel=2)
 
 
-def _warn_wqx3_unavailable():
+def _warn_wqx3_unavailable() -> None:
     # stacklevel=3: warn -> _warn_wqx3_unavailable -> _legacy_only_url -> what_*
     warnings.warn(
         "WQX3.0 profile not available, returning legacy profile.",
diff --git a/pyproject.toml b/pyproject.toml
index 62ac7478..57f60161 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -32,6 +32,11 @@ packages = ["dataretrieval", "dataretrieval.codes"]
 dataretrieval = ["py.typed"]
 
 [project.optional-dependencies]
+# Minimal set the CI ``type-check`` job installs — just mypy + the package,
+# not the whole test stack.
+type-check = [
+  "mypy<2",  # <2 so it can still target Python 3.9 (the project's floor)
+]
 test = [
   "pytest > 5.0.0",
   "pytest-cov[all]",
@@ -39,6 +44,7 @@ test = [
   "coverage",
   "pytest-httpx",
   "ruff",
+  "dataretrieval[type-check]",  # mypy, pinned once in the type-check extra
 ]
 doc = [
   "docutils<0.22",
@@ -102,3 +108,14 @@ skip-magic-trailing-comma = false
 line-ending = "auto"
 docstring-code-format = true
 docstring-code-line-length = 72
+
+[tool.mypy]
+# The package is fully annotated and passes ``mypy --strict``. The one
+# remaining relaxation is ``ignore_missing_imports``: untyped third-party
+# libraries (pandas, geopandas, anyio) are treated as ``Any`` instead of
+# requiring stub packages. Dropping that — via pandas-stubs/types-requests and
+# per-module overrides — can follow.
+python_version = "3.9"  # the project's minimum supported version
+files = ["dataretrieval"]
+strict = true
+ignore_missing_imports = true