TauDEM

TauDEM (Terrain Analysis Using Digital Elevation Models), by David Tarboton at Utah State University, is the research-grade standard for DEM-based hydrology: pit removal, D8 and D-infinity flow models, contributing area, and stream-network delineation with objective drop-analysis thresholds rather than arbitrary ones. Its MPI parallelisation lets it chew through very large DEMs.

The D-infinity method — apportioning flow between neighbours by slope direction — originates here and matters wherever dispersal and wetness patterns count. Interfaces exist for ArcGIS and QGIS, and the CLI slots into scripted pipelines cleanly.