Arc Hydro
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.

500+ fast geospatial analysis tools with the best DEM hydrology toolkit in open source.
by Whitebox Geospatial Inc.
Install
pip install whiteboxWhiteboxTools, from John Lindsay's Geomorphometry and Hydrogeomatics lab at the University of Guelph, packs over five hundred geoprocessing tools into a single fast Rust binary — and its hydrological terrain analysis is the deepest available anywhere: depression breaching (its least-cost breaching is the reference implementation), flow pointers and accumulation in many flavours, stream extraction, wetness indices and lidar processing.
It runs standalone, from Python, or inside QGIS/ArcGIS via frontends, with no dependencies to install. For DEM conditioning and delineation pipelines, WhiteboxTools is usually the sharpest free tool in the drawer.
The free GIS stack for going from raw DEM to watershed boundary and stream network.
Ranked by the topics, categories and workflow stages they share with WhiteboxTools.
Esri's free hydrology framework for ArcGIS Pro — terrain preprocessing and the data model behind many agency workflows.
Four decades of geospatial engineering — r.watershed and the most battle-tested hydrology modules in GIS.
The global hydrography reference — basins, river networks, lakes and hydro-environmental attributes, free.

The geomorphometry powerhouse — hundreds of terrain and hydrology modules behind a free desktop GIS.
Utah State's parallel terrain-analysis suite — D-infinity flow modelling and objective stream-network extraction.
Watershed delineation in pure Python — DEM conditioning to catchment boundary in a dozen lines.