**06/03/2026:** The request-error hierarchy is now unified. Every module (`nwis`, `wqp`, `nldi`, `waterdata`, `nadp`, `streamstats`) raises a subclass of `dataretrieval.DataRetrievalError` on a failed request, so a single `except dataretrieval.DataRetrievalError` spans them all. An HTTP error status surfaces as an `HTTPError` carrying `.status_code` (inspect it to branch on a specific code); the retryable 429/5xx subset is `TransientError` (`RateLimited` / `ServiceUnavailable`, carrying `.retry_after`); and a request too large to satisfy is a `RequestTooLarge` (`URLTooLong` for an over-long single request, `Unchunkable` when the Water Data chunker cannot split a call small enough). Connection-level failures (timeouts, DNS, refused connections) are wrapped as a `NetworkError`, with the underlying `httpx` exception on `__cause__`. Every `DataRetrievalError` also exposes `.status_code` (`None` when there is no HTTP status), `.retry_after`, and `.retryable`, so a single `except dataretrieval.DataRetrievalError as e` clause can branch on the status or retry transient failures without knowing the concrete subclass. **Breaking change:** these exceptions no longer multiply-inherit a built-in — code that caught request failures with `except ValueError` or `except RuntimeError` should switch to `except dataretrieval.DataRetrievalError` (or a specific subclass). A no-data result is **not** an error: the modern getters (`waterdata`, `wqp`, `nldi`) return an empty DataFrame when nothing matches. Only the deprecated `nwis` (waterservices) path still raises `NoSitesError` on no data.
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