Quoted From : https://hydrology.usu.edu/taudem/taudem5/help53/LengthAreaStreamSource.html

    Creates an indicator grid (1,0) that evaluates A >= (M)(L^y) based on upslope path length, D8 contributing area grid inputs, and parameters M and y. This grid indicates likely stream source grid cells. This is an experimental method with theoretical basis in Hack's law which states that for streams L ~ A^0.6. However for hillslopes with parallel flow L ~ A. So a transition from hillslopes to streams may be represented by L ~ A^0.8 suggesting identifying grid cells as stream cells if A > M (L^(1/0.8)).