BOARD00038 ‎(2)‎

• Board Meeting Date: 9/20/2006
• Description: WSP Section - San Juan River Recovery Implementation Program Document Revisions
• Board Meetings - Doc Type: Memo

CHAPTER 2: SAN JUAN RIVER BASIN

The San Juan River Basin is the second largest of the three sub-basins which comprise the Upper
Colorado River Basin (Figure 1). The San Juan River Basin drains about 38,000 square-miles of
southwestern Colorado, northwestern New Mexico, northeastern Arizona, and southeastern Utah.
From its origins in the San Juan Mountains of Colorado, the San Juan River flows some 31 miles to
the New Mexico border, 190 miles westward to the Four Comers area, and then another 136 miles
to Lake Powell. In its upper reaches, the river traverses rugged terrain and has a relatively high
gradient. The river emerges from canyon-bound reaches at Navajo Reservoir in New Mexico and
flows through a broad floodplain for much of its course in New Mexico and Utah. About 70 miles
upstream of Lake Powell, the river again enters canyon reaches for the remainder of its course. The
river is generally restricted to a single channel in canyon portions, but is often divided into several
channels in floodplain reaches.

The San Juan River has comparatively few perennial tributaries, most of which are in upper
reaches. Historically, the Rio Blanco, Navajo River, Piedra River, Los Pinos River, Animas River,
La Plata River, Mancos River and McElmo Creek were the only perennially flowing tributaries.
Other streams such as Montezuma and Chinle Creeks contributed flows seasonally. Numerous
washes and arroyos also entered the river, but none provided regular flow. The Animas River
contributes the greatest flow.

Historic Flows

The San Juan River exhibits highly variable annual and monthly natural flows. Prior to the
construction of Navajo Dam, the hydro graph was characterized by large spring peaks resulting
from snowmelt runoff and low base flows at other times of the year. Typically, spring runoff begins
in March, peaks in mid-May to early June, and ends by the first week of July. The remainder of the
year, natural flow is characteristically low, punctuated by large, short-duration peaks caused by
summer and fall storm events. Measured flows in the San Juan River at Bluff, Utah, have ranged
from a high of about 70,000 cubic-feet-per-second (cfs) to no flow at times.

Since the completion of Navajo Dam in 1962, flows immediately below Navajo Reservoir and
above the Animas River confluence have been largely controlled and stabilized. Prior to the
initiation of experimental flow releases from Navajo Dam in 1991 to meet needs of the endangered
fish species in the San Juan River, spring peak flows were significantly reduced in magnitude, base
flows were increased and stabilized, and late winter flows were increased markedly to provide
storage space in the reservoir for the spring runoff. Since 1991, the reservoir has been operated to
mimic a natural hydrograph with high spring peak releases of about 5,000 cfs, which is the
downstream channel capacity, and low base flow releases. In addition to moderating natural flows,
hypolimnetic releases from Navajo Reservoir have decreased mean annual water temperature and
diminished temperature fluctuations of the San Juan River downstream to near the confluence of
the Animas River.

From Navajo Dam to Lake Powell, flows in the San Juan River are supplemented by inflows from
perennial and ephemeral tributary streams, arroyos and washes. Diversions of flow of the San Juan
River and its tributaries are made for industrial, municipal and agricultural uses. Inflows below
Navajo Dam provide the main variation in flow, including spring runofIand thunderstorm peaks.

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