Hydrology of Area 61, Northern Great Plains and Rocky Mountain Coal Provinces, Colorado and New Mexico

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6.0 SURFACE -WATER QUALITY -- Continued 6.3 Sediment -- Continued 6.3.2 Sediment Yields Sediment Yields Are as Much as 2,840 Tons Per Square Mile Per Year Computed annual suspended- sediment yields at four stations in the study area ranged from 300 to 2,840 tons per square mile per year. The streamflow at eight streamflow- gaging sta- tions in Area 61 was sampled for suspended sedi- ment. However, sufficient data for analysis are available only for stations 8 (Apishapa River at Aguilar, Colo.), 10 (Molina Canyon near Weston, Colo.), 14 (Reilly Canyon at Cokedale, Colo.), and 16 (Carpios Canyon near Jansen, Colo.) (fig. 6.3.2 -1). The geology of the areas upstream from these four stations is similar (fig. 6.3.2 -1); therefore, differences in sediment yield and sediment discharge cannot be attributed to variations in geology. Area 61 is characterized by a large range of sediment yields and instantaneous suspended -sedi- ment discharges. The sediment yield in the area ranged from 300 to 2,840 tons per square mile per year, and measured instantaneous suspended -sedi- ment discharge ranged from 0.003 to 612,300 tons per day. These large yields and discharges probably are due to a combination of basin characteristics, such as the semiarid climate and erodible soils. 58 Relations between water and suspended -sedi- ment discharge such as those shown in figure 6.3.2 -2 can be used to estimate sediment loads for water discharges of magnitudes within the range of the relations. These relations were developed from data collected at the four streamflow - gaging stations dur- ing both high and low flows. _By using the daily streamflow record at each of these stations and the relations in figure 6.3.2 -2, the annual sediment yield can be estimated for each of the respective drainage basins. The yields given in figure 6.3.2 -2 are averages determined from 4 years of record that were available and show that sediment yield decreases as drainage area increases. This relation is a result of the in- creased annual production of sediment with decreas- ing drainage area and the more efficient transport of this sediment to the stream channel by overland flow in the smaller drainage basins. This conclusion also has been reached in studies of other semiarid areas of the West (Brune, 1950. Hadley and Schumm, 1961).