7370 - Laserfiche WebLink

Home Browse Search

Utah: RM 136 to RM 0 The San Juan River traverses several different physiographic regions as it courses through Utah. In the uppermost reach, from the U.S. Highway 160 Bridge (RM 136; elevation 1,423 m (4,669 ft]) to Chinle Creek (RM 78; elevation 1,285 m [4,216 ft]), the river was a low gradient (1.3 m/km [6.8 ft/mi]), relatively shallow, braided and meandering stream in a broad floodplain. The only major tributary in this reach was Montezuma Creek. The next section of the Utah portion of the study area was a relatively short reach (18 mi, 29 km) from Chinle Creek (RM 78) to Mexican Hat (RM 60). In this region, the morphology of the river changed from a braided network within a sandy floodplain near Chinle Creek to a single channel as it entered San Juan Canyon. Approximately 7 miles (11 km) upstream of Mexican Hat (11 mi; 18 km downstream of Chinle Creek.), the canyon widens and the river enters the geologic region known as Soda Basin. This basin was created by the dissolution of evaporite formations and caused broadening of San Juan Canyon (Huber 1974). In the 44 mile (71 km) reach of the San Juan River from Mexican Hat (RM 60) to Grand Gulch (RM 16), the third distinct geomorphologic area, the river was confined to a single, relatively uniform channel within the deep, narrow canyon area of the Monument Upwarp. Riverine habitat was comprised primarily of deep runs, but occasional rapids created by boulders were also present. River substrate was mainly bedrock or sand-silt. Salt cedar and willows were present in limited numbers throughout the canyon reach and were restricted to the inside curves of river meanders. River elevation decreased from 1,235 m (4,052 ft) at Mexican Hat to 1,135 m (3,727 ft) at Grand Gulch, a gradient of 1.4 m/km (7.4 ft/mi). The downstream-most section of the study area encompassed the 16 mile (26 km) reach from Grand Gulch to Piute Farms Marina. This section of the San Juan River, although influenced by Lake Powell water elevations, generally retained most of its riverine characteristics. The river was contained within steep-sided canyons in a single channel. River banks had limited riparian vegetation. The river had relatively swift current and a substrate comprised of sand and sandstone slabs. Temporary backwaters formed during periods of reduced flow. San Juan River Discharge Discharge in 1988 and 1989 was considerably less than that of 1987 (Figure 7). For example, spring discharge at the Farmington gauge during May 1987 was 6.5 and 5.4 times greater, than that recorded in late April 1988 and early May 1989, respectively (Table 1). October flows at the Farmington gauge in 1987 were 1.6 times as great as those in 1988 and 1.2 times greater than October 1989 discharge. 16