Strahler Stream Order

Stream Order - Stream order (calculated in the NHD using Strahler Stream Order (Strahler, A.N., 1952; Horton, R.E., 1945)) in hydrography deals with the hierarchy of streams from the source (or headwaters) downstream. The NHD is capable of storing stream order information in the NHDFlowlineVAA table. However, since stream order must start at the headwaters of the features, and most subbasins (8-digit hydrologic unit containers) do not contain the headwaters , this attribute is not populated


The headwaters are the first order and downstream segments are defined at confluences (two streams running into each other). The diagram below shows how the downstream numbering of stream order is done. At a confluence, if the two streams are not f the same order then the highest numbered order is maintained on the downstream segment. At a confluence of two streams with the same order, the downstream segment gets the next highest numbered order. Divergences such as braided streams actually maintain the same order all the way through the braid, just like it was a single stream; however, divergences that are not braided streams keep the upstream order number and follow the normal hierarchy further downstream.


Horton, R. E. (1945), "Erosional development of streams and their drainage basins: hydro-physical approach to quantitative morphology", Geological Society of America Bulletin 56 (3): 275–370


Strahler, A. N. (1952), "Hypsometric (area-altitude) analysis of erosional topology", Geological Society of America Bulletin 63 (11): 1117–1142