Chapter 2--Area Description and Resources
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Chapter 2—Area Description and Resources © Keith Penner Yellow-headed Blackbird when the previously deformed Precambrian (over 570 Physical Environment million years old) and Paleozoic (570–240 million years old) rocks were slowly uplifted and broken into huge The physical environment comprises the geology, fault blocks by extensional stresses that still continue soils, hydrology, and climate of the Bear River water- to stretch the earth’s crust (Milligan 2000). shed. In addition, climate change is discussed. Sediments shed from the ranges are slowly filling the intervening wide, flat basins. Many of the basins have been further modified by shorelines and sed- Geology and Soils iments of lakes that intermittently cover the valley floors. The most notable of these was Lake Bonn- The Bear River basin encompasses two physio- eville, which reached its deepest level about 15,000 graphic provinces: The Basin and Range Province and years ago when it flooded basins across western Utah the Middle Rocky Mountain Province of the Rocky (Milligan 2000). Mountain Section (Dion 1969). The Basin and Range The Middle Rocky Mountains Province, which Province is noted for many north–south oriented, encompasses approximately the eastern two-thirds fault-tilted mountain ranges separated by interven- of the basin, consists of mountainous terrain, stream ing broad, sediment-filled basins. Approximately valleys, and alluvial basins. The Utah part of this the western one-third of the watershed lies within province has two major mountain ranges, the north– the Basin and Range Province, which began forming south trending Wasatch and east–west trending 10 Land Protection Plan—Bear River Watershed Conservation Area; Idaho, Utah, Wyoming Uinta Mountains. Both ranges have cores of old Pre- vary in elevation from 7,500 to 13,500 feet and are cambrian rocks, some more than 2.6 billion years old unusual in that they run in an east to west orien- (Milligan 2000). This Precambrian bedrock became tation. From the headwaters, the Bear River flows exposed during the Pleistocene by glacial activity north and west in an arc from Utah, Wyoming, Idaho, that created smooth bowls that collect and funnel and back into Utah. Near the city of Evanston, Wyo- water down the Bear River (Denton 2007). ming, the topography flattens and land use becomes The Bear River Range, located in the central a mix of urban and agricultural uses. Here the river part of the Bear River watershed, is aligned north begins a dramatic transformation from fast-flow- to south and divides the eastern Mesozoic and west- ing, cold, and clear water in the narrow valleys to a ern Cenozoic zones. From the Uinta Mountains in the slow-moving, cool-water, meandering course on the eastern part of the watershed, the Bear River flows valley floor. Humans have altered the natural stream northward along the edge of a Mesozoic region, char- dynamics throughout the remaining course of the acterized by rock structures that have little ability to Bear River to its termination at the Great Salt Lake. absorb water. The western part of the watershed is Although agriculture accounts for only 7 percent of comprised primarily of Paleozoic rock in the moun- the land use in the upper watershed, it accounts for tains and Cenozoic rock in the valleys. The valleys more than 80 percent of the water usage. Surface and here contain alluvial and glacial deposits that are ground water sources are used to irrigate more than absorptive and lend well to agricultural use (Haws 96,512 acres of hay, pasture, and cropland (Bear River and Hughes 1973). The Bear River range is an impor- Watershed Information System 2009). tant catch basin for precipitation. Instream structures like the Chapman Canal The watershed contains multiple mountain ranges Diversion and Woodruff Narrows Reservoir dis- including the Wasatch Front to the west, the Bear rupt natural channel-forming flows and sediment River Divide (Crawford) and Tunp Ranges to the transport, leading to streambed and bank instability east, and the Sublette Range to the north (see fig- downstream. After passing through Woodruff Nar- ure 3). The convergence of mountain ranges at Rocky rows Reservoir, the valley broadens and the river Point about 1 mile northeast of Cokeville creates a travels along the Wyoming–Utah border and lends pinch-point for one of the regionally important migra- itself to irrigation and production agriculture for 30 tion corridors in the watershed. The position and miles before reentering Wyoming near Sage Junc- alignment of the various ranges across the watershed tion. Nutrient loading (especially phosphorus, which play a central role in precipitation, climatic, hydrolog- is found at naturally high levels in surrounding soil ical, and biological patterns. formations), sediment from accelerated bank erosion, and dewatering are leading causes of stream degra- dation. Sediment and nutrient levels remain as the Hydrology main water quality concerns throughout the entire Bear River watershed, and those impacts contribute The Bear River is the largest tributary to the to water management challenges in the refuges (Utah Great Salt Lake, the remnant of ancient Lake Bonn- Division of Water Resources 2002). eville. Lake Bonneville was a closed inland sea basin As the river flows north from Evanston, the ridge the size of Lake Michigan that once dominated the and swale topography of the floodplain is character- landscape in Idaho, Nevada, and Utah. Approxi- ized by a complex association of irrigated meadows, mately 16,000 years ago, Lake Bonneville began wetlands, and grass uplands that support one of the spilling over into the Snake River drainage at Red highest densities of migrating and nesting waterfowl Rock Pass, reducing the lake level by 375 feet. Over in Wyoming. Centered along a 20-mile stretch of the the following 8,000 years, Lake Bonneville contin- Bear River and its associated wetlands and uplands, ued to shrink because of changing climatic conditions, Cokeville Meadows National Wildlife Refuge was eventually occupying only the present day Great Salt established in 1992 to protect this important habitat. Lake (Utah Geological Survey). After leaving Cokeville, the Bear River crosses The Bear River watershed is unusual in that it into Idaho near the community of Border, where the is entirely enclosed by mountains, forming one arm flow is greatly increased by inflow from the Smith’s of the Great Salt Lake basin, which has no natural Fork River, which originates in the Bridger–Teton drainage outlets. Three States share drainage in the National Forest and has a relatively intact water- 7,500 square-mile watershed: 2,700 square miles in shed and native fish assemblages (Wyoming Game Idaho, 3,300 square miles in Utah, and 1,500 square and Fish Department 2010). miles in Wyoming. Progressions of small, high-moun- As the Bear River passes into Idaho, Pacifi- tain streams form the headwaters of the Bear River Corp diverts water at Stewart Dam through Bear in Utah’s Uinta–Wasatch–Cache National Forest. The Lake National Wildlife Refuge and into Bear Lake Uinta Mountains, a subrange of the Rocky Mountains, proper (which straddles Idaho and Utah). Bear Lake Chapter 2—Area Description and Resources 11 Figure 3. Vicinity map for the Bear River Watershed Conservation Area in Idaho, Utah, and Wyoming. 12 Land Protection Plan—Bear River Watershed Conservation Area; Idaho, Utah, Wyoming National Wildlife Refuge, near Montpelier, Idaho, was established in 1968 to protect and manage habitat for Climate waterfowl and other migratory birds. Once released from Bear Lake proper, water travels from the Out- The climate of most of the Bear River Water- let Canal and the refuge’s Mud Lake unit back to shed Conservation Area is characterized as having the Bear River’s original channel about 7 miles from warm to hot summers and cold winters and is clas- where the water is first diverted. Except for some sified as humid continental, mild summer under the water seepage from Stewart Dam, all Bear River Koppen climate classification system. The remainder water is diverted through the refuge; however, small of the watershed near the Great Salt Lake is classi- creeks and irrigation return water enter into the orig- fied as semiarid desert–steppe or humid continental, inal river channel so that the river is not completely hot summer for the Great Basin and Wasatch Front, dewatered between Stewart Dam and its reunion respectively. with the Outlet Canal. Annual precipitation is influenced greatly by the From Bear Lake, the river travels 100 miles to the topography and elevations found within the water- north, where it is impounded in the Alexander Res- shed, which range from 4,200 to 13,000 feet. Annual ervoir for irrigation, recreation, and hydroelectric precipitation ranges from 10 inches in the lower val- power generation. Below the Alexander Dam, about leys to 65 inches at the headwaters of the Bear River one-tenth of the river’s annual flow is sent through in the Uinta Mountains (Utah Division of Water one of the oldest diversion canals in the watershed, Resources 2005b). Two major storm patterns influ- the Last Chance Canal. The canal was constructed ence precipitation in the basin: (1) frontal systems by settlers to provide irrigation for agriculture in the from the Pacific Northwest during winter and spring; early 1900s. From there, the river continues south and (2) thunderstorms from the south and southwest toward Grace, Idaho. Just above the Black Can- in late summer and early fall. yon, almost all the river water is again diverted, at Temperatures are also variable throughout the the Grace Dam, through an aqueduct to the Grace watershed because of differences in elevation. Mean Power Plant for power production. The water then is annual temperatures range from 37 °F in the Uinta returned to its original river channel just below Black Mountains at about 8,400 feet elevation to 53 °F at Canyon at Cove Dam.