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i i discharge at Line Camp is 12,400 cubic feet per second (USAGE, 1978) the Army Corps' velocity estimates correlate well with the Division of Minerals and Geology estimates distributed at the Line Camp Hearing before that Mined Land Reclamation Board, and also correlate with the velocities measured at the USGS gauge in Dolores during high flow events. The over bank velocity during a flood at Line Camp may be attenuated to well below 4 feet per second due to the very shallow floodwater and the presence of a cottonwood grove in the buffer zone between the excavations and the river. Flow velocities of 4 feet per second and floodwater depths of less than a foot to 1.5 feet are not capable of down cutting in the buffer zone left between the pits and river. Velocities of 5-7 feet per second are requ'ved to cause erosion of loose material, such as stream banks and bridge abutment fill. Velocities of 10 feet per sewnd or more are required to cause severe erosion of channels, destroy low water crossings, and transport boulders. Deposition of suspended load will begin when velocities drop to 2 feet per second or less. The 100-year flood flows in the river channel adjacent to the Line Camp Pit will be approximately 8 feet deep (LJSACE, 1978). These flows have the potential to cause riverbank erosion, but the minimum 200-foot buffer strip between the pits and the river is more than sufficient to prevent bank-caving progression from intercepting the pits. This is particularly true since the river is very straight through the Line Camp reach and the proposed pits are to be located on the inside of a very slight bend in the river. Parties concerned with the Line Camp application have stated that the proposed temporary pit perimeter berms will redirect flows during a flood. Based on review of the flood depths at pit perimeters as described above, the proposed temporary berms can be expected to intersect shallow, low energy water during a 100-year flood. The flood will essentially lap up against the base of the berms. The approved permit application requires that gaps be installed in the berms to allow the floodwater to pass through without any increase in base flood surface elevation. The Division of Minerals and Geology has analyzed the potential impacts of the excavation of the Line Camp Pits on the behavior of a 100-year flood on the Dolores River. Portions of the proposed excavations aze located in the shallow, low energy fringe of the 100-year floodplain. Floodwater will flow into and through portions of the excavations during a flood, then retreat from the excavated area back toward the existing channel without causing any adverse offsite impacts related to the mining of the pits. 3