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. CHAPTER 2. EXISTING CONDITIONS AT THE MA 2.1 Hydrology The MA is located in the eastern Colorado Plateau semi desert province. The La Plata River originates in the La Plata Mountains about 25 miles northwest of Durango, Colorado and generally flows southward to its confluence with the San Juan River near Farmington, New Mexico. Average precipitation for the MA is about 10-12 inches a year. The hydrologic characteristics of the La Plata River have a major influence on the presence of wetland/riparian plant communities along the river corridor. Because of the river, the floor of the valIey bottomland has a moist environment capable of supporting wetland/riparian plant communities, whereas the adjacent tablelands, in the absence of irrigation water, can only support semiarid plant communities. River flows and irrigation return flows recharge the alIuvial water table and periodically inundate the floodplain, creating sediment deposits that provide nursery habitat for the recruitment of wetland/riparian plants. Within the MA, the La Plata River has perennial flow due to the combined inflow of the La Plata River, Cherry Creek, Long HolIow, groundwater recharge, and several tributary ephemeral streams draining irrigated lands east of the river. However, summer flows within the corridor are very low and range from about 0 to 4 cfs between Cherry Creek and Long HolIow, and about 8 cfs immediately below Long HolIow. Flow in the La Plata River is characteristic of many western rivers that have watersheds containing both a semiarid plains and high mountain headwaters. The hydro graph typically crests during springtime melting of the mountain area snowpack. Irrigation diversions do influence the hydro graph by lowering the peak of high flows and generally reducing the river flows throughout the irrigation season. Streamflow is generalIy low from mid-summer until the folIowing year's spring runoff event. RainfalI-induced floods that originate as overland runoff in the watershed's lowlands generally occur from July through October. These flood events have high peak-discharge flows but are short-lived, with the entire flood event usually lasting less than one or two days, and sometimes just a few hours. Late- summer floods typicalIy have higher instantaneous peak flows than spring snowmelt floods. High flow conditions on the La Plata River produce large suspended sediment loads and turbid water. River flow and irrigation return flow are also indicators of depth-to-groundwater in the valley's alluvial bottom land. The shalIow water table within the valIey's zone-of-influence provides a ready source of moisture for water-loving (phreatophytic) species associated with wetland/riparian plant communities. The zone-of- influence is defined as that area within the river valIey that is influenced by the river's hydrology, both surface water and ground water. TypicalIy, the zone-of-influence supports wetland/riparian plant communities that are dependent on the hydrology and geomorphic processes of the river. As described below, the river valIey has four distinct alIuvial surfaces that were formed by the geomorphic processes of the river. These surfaces have differing capabilities for supporting wetland/riparian vegetation because of different flood frequencies and depth to the underlying water table. -11-