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seasonal river crossing and the operations al the Cuuuc use area; however, the (lows arc generally within the design capacity of the seasonal road crossing. Table 2. Ranges of Bows in the Animas River at Durango for winter mon(hs (period of record 1897 - 2003) Nov Dec Jan Feb Mar Minimctnt 758 729 103 170 133 Mean 285 222 202 205 296 Mabmum 814 412 326 352 844 Reclamation of the Future Use Area Reclamation of the future use area will proceed concurrently with the mining (Sandco, Inc., Permit Application, Regular 112, Amendment, April 18, 2004). As backslopes are created and graded to 1 ft vertical to 3 ft horizontal, the adjacent previously stockpiled topsoil will be graded over the slopes to the waters edge. Thickness of the topsoil will range from 6 [0 12 inches over alt reclaimed slopes. A mixture of native and preferred agriculture vegetation will be planted. L'rosion control measures will be imple- mented. EFFECTS OF FLOW REGIME ON THE ANIMAS RIVER CHANNEL AND THE FUTURE USE AREA The future use area, being located off-channel, will likely have less impact on the Animas River channel than the in-channel mining. However, there have been many concerns expressed about the over- flow of floods into the future use lake and the effects of floods on erosion of the existing channel. Floods Floods can carry tremendous power, and can have devastating effects on land, buildings, river banks, and flood plains. River channels are constantly changing course, especially in high energy systems such as the Animas River. An example of [he frequent changes in river course is exhibited by the surficia! geologic map of the Animas River valley near Trimble Lane (Blair and others, 2002). The geologic map shows oxbows where the Animas River once (lowed- Also shown in figure 7 is a photograph of an oxbow lake in the Animas River valley. The Animas River valley has many old abandoned channels and oxbows. This indicates that the river changes course quite frequently. The ages of the oxbow lakes in the Animas River valley may date to the late 1890's (Church and others, 1999), indicating that the river has changed course sometime behvicen the 1890's and the prCSerll. Concerns have been expressed that the Animas River during a flood will overtop its banks and flow into the Sandco future use lake, thereby changing the course of the Animas River. From the anecdotal and scientific information, the Animas River changes course frequently, and it does not necessarily need a man-made lake to affect its change of course. Modeling the 100-year flood The computer model hIGC-RAS (U.S. Army Corps of Engineers, 2002) was applied to the Sandco Reach of the Animas River to simulate the water- surface elevations at steady-state conditions for different flow regimes (].5-, 10-, 50-, and 100-year floods). The modclin~ system calculates water- surface profiles for steady gradually varied flow. The steady flow component is capable of modeling subcrit- ical, supercritical, and mixed flow regime water- surface profiles. The basic computational procedure is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction losses (Manning's equation) and contraction/expan- sion (coeticient multiplied by the change in velocity head). The momentum equation of utilized in situa- tions where the water-surface profile is rapidly varied. These situations include mixed flow regime calcula- tions (i.e, hydraulic jumps), hydraulics of bridges, and evaluating profiles at river confluences. Surveyed cross-section data for the Sandco Reach were collected for all river stations (SC-1 through SC-16) by Animas Surveying and Mapping during November, 24-26, 2002 (BLQ, Inc., written commun., 2004). More recent cross-section data are available for selected river stations; however, the most complete data set exists for November 2002. Values for Manning's coefficient (n values) were selected from the U.S. Geological Survey Water Supply Paper nn Roughness Characteristics of Natural Channels (Barnes, 1967). Eor the main channel, a roughness Hydrologic Study of the Sandco Reach, Animas River Valley, La Plata County, Colorado, 2004 8