Overview
Lake Lowell, originally known as Deer Flat Reservoir, is an offstream reservoir formed by three earthfill dams enclosing a natural depression southwest of Nampa, Idaho. These three dams are the Upper, Middle (Forest Dam), and Lower Embankments. A fourth embankment called the East (Roadway Dike) is to protect farmsteads on the eastern end of the reservoir when the reservoir is full. The reservoir is filled primarily during the non-irrigation season by diversions at the Boise River Diversion Dam and conveyance through the New York Canal which discharges into the eastern (upper) end of Lake Lowell.
The Upper Embankment on the north side of the reservoir is65 feet high and has two outlets; near the right abutment is the Deer Flat Nampa Canal (100 cubic feet per second) and near the left abutment is the Deer Flat Caldwell Canal (70 cubic feet per second).
The Lower Embankment at the west end of the reservoir is 41 feet high. The Deer Flat Low Line Canal (1,200 cubic feet per second) outlets are on the left abutment and the Deer Flat North Canal (70 cubic feet per second) outlets are on the right abutment.
The Middle Embankment, which has no outlets, is 12.7 feet high and helps to close the reservoir near the Lower Embankment. It has been referred to as an emergency spillway; however, no slope protection is provided for this purpose.
Because ofsafety concerns at the Upper and Lower Embankments, the maximum water level was reduced by 5 feet in 1989 while plans were prepared to address remedial actions. Major modifications were made to the Upper and Lower Embankments from 1991 to 1993. Further, in 1996 an upstream seepage blanket was constructed at the right abutment of the Upper Embankment to address seepage in this area. These activities were accomplished under Reclamation’s Safety of Dams Program. A Safety of Dams modification was also performed in 2008-2009. The modifications involved the complete replacement of the Caldwell Canal outlet works structure including the intake tower and access bridge, gate and operator, the entire length of conduit, and the placement of a circumferential filter around the downstream section of new conduit. In addition, the downstream section of the Nampa Canal outlet works conduit was replaced with a new section of conduit incorporating a downstream circumferential filter.
The original total storage capacity of Lake Lowell was 190,000 acre-feet (active 169,000 acre-feet). A sedimentation survey conducted in 1994 estimated the reservoir’s total storage capacity at 173,000 acre-feet (active 159,400 acre-feet).
Geology
Deer Flat Reservoir (Lake Lowell) is an off stream storage site that was created by damming three low areas in a natural broad channel. The natural channel is probably an old drainage way or glacial flood channel. The dams and reservoir are located within a large alluvial filled basin, which is underlain by hundreds of meters on unconsolidated to slightly consolidated sediments. Most of the sediments are fluvial but some are lacustrine in origin. In some parts of the basin there are intercalated basaltic lava flows. The formation directly underlying the damsite is called the Caldwell-Nampa sediments of the Pleistocene age. This formation, which is up to 15m (50ft) thick, is described as unconsolidated layers and lenses of clay, silt, sand and gravel. Beneath the Caldwell-Nampa sediments, and outcropping in some places near the reservoir, is the Ten-Mile Gravel Formation which is described as up to 152m (500ft) of poorly consolidated silt, sand, gravel and cobbles. Overlying these sediments are scattered, recent thin deposits of sand, gravel and windblown silt.
Other Dams or Dikes which enclose Lake Lowell -Deer Flat Dams consist of four earthen embankments at separate locations forming the offstream Lake Lowell Reservoir -1) Deer Flat Upper Embankment, 2) Deer Flat Lower Embankment, 3) Deer Flat MiddleEmbankment, and4) Deer Flat East Dike.
