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Hydrologic Study of the Sandco Reach, Animas River Valley, La Plata County, Colorado, 2004 By Winfield G. Wright, Southwest Hydro-Logic Abstract The Sandco Reach ofthe Animas River has been a location for in-channel gravel mining since 1986. As an alternative to in-channel mining, Sandco plans to operate an ofl=channel gravel mining area where the work will occur during the winter months of November through March. A lake will be created as the operation proceeds, and the lake area will be enlarged by 2 acres per year for 20 years. The new lake area may be converted to a conservation easement when the work is completed. Concerns have been expressed by regulatory agencies and citizens about the hydrology of the Animas River in the Sandco Reach. This report addresses concerns relating to construction of the lake, hydraulics of the seasonal river crossing, effects of different flow regimes and floods on the channel and lake, and the effects of the future use on water quantity, water quality, and erosion. INTRODUCTION Because of the high quality of sand and gravel, the Animas River channel and overbank deposits in the northern part of the valley (figure 1) have been utilized for many years by aggregate companies. The sand and gravel from this area has supported the main- tenance and development of La Plata County and the City of Durango. Much of the gravel mining has occurred as in-channel operations under the hypothesis that the gravel would be replenished each year by high altitude snowmelt runoff from the San Juan Moun- tains. Since about 1999, there has been a recurring drougfitin Colorado and throughout the western United Slates; therefore, the annual snowpack has been below average and snowmelt runotl'dischazges have not been sufficient to transport large quantities of sediment and replenish the gravel resources. Subse- quently, the Animas River channel has exhibited impacts from the drought. These impacts include channel incision, bank erosion, possible lowering of the water table beneath the valley floor, loss of riparian habitat, reduced overbank flood depths near the gravel pits, and increased overbank flood depths farther down the valley (Gillam, 2003). As an alternative to the in-channel gravel mining, Sandco, Ina plans to develop an off-channel sand and grave( resource where the work will occur only during the winter months, a lake will be created from the gravel mining, and the lake will be enlarged by 2 acres per year for 20 years. The new area may be converted to a lake and wetland conservation ease- ment when the work is completed. There generally has been approval of the oti channel gravel mining concept because i[ does not directly impact [he channel and riparian areas, and upon completion the lake will provide wildlife habitat and open space for parks and recreation. Concerns have been expressed by regulatory agencies and citi- zens relating to the reclamation of the historical use area, hydrology of the Animas River reach, construc- tion of the lake, hydraulics of the seasonal river crossing, effects of different flow regimes and floods on the channel and lake, and the effects of the future use area on water quantity, water quality, and erosion. This report addresses the above concems as best as possible using available data. No new data were collected related to the hydrology, climate, or riparian habitat. New interpretations ar'e provided in this report including hydraulics of the seasonal river crossing, modeling of the river reach to simulate different flow >~ Southwest Hydra-Logie t W ~__,.