Navajo Lake. Fed by the Pine, Piedra, and San Juan where the first two named feed the San Juan. The San Juan flows out of Navajo and feeds Powell directly. Before Powell though, the Animas and La Plata feed the San Juan. Navajo fuels one of the largest agricultural projects in the country and is located on the Navajo Reservation.
The reservoir was built in the late 1960's for that very reason. The town of Rosa, NM was buried under water near the southern border of CO near the mouth of Eul Canyon (great smallmouth fishing there by the way). Unit 2B in Rio Arriba County is on the southern shore of the lake as it runs nearly 30 miles from the dam upstream into CO.
Severe drought hit NW NM and SW CO in 2004-2005 (and maybe in 2003, I can't remember exactly and don't really care to look it up, but you can if you want) and had very little snowpack and spring run-off. The "down-streamers" cried too much, and water was sent.
NAPI got its allotment no matter what - welcome to tribal politics. Phoenix would burn up and blow away before a tribe is required to give up its water rights.
Also, in about 2010 or so, high discharges were sent downstream to Powell most of the summer because UT and WY just didn't have the "world's greatest snow on earth" that year and levels were low.
Navajo Lake State Park
NAPI
So, bob, does that shed enough light on it for you? Sorry to burst your bubble, but those are the facts...