It is feasible to ask, and relatively easy to answer. The San Juan river flows from the dam at Navajo Lake(Northeast of Aztec, NM), and travels primarily through the Navajo nation(picking up flows from the Animas River, La Plata River, Mancos River, and McElmo Creek(not a significant amount of water, but worth a mention). Historically speaking, any ravine has been used as a convenient "dump", where gravity helps do the work of unloading the truck. When high water comes up, instant cleanup. It just happens to end up in Powell.