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6.0 SURFACE -WATER QUALITY -- Continued <br />6.3 Sediment <br />6.3.1 Sediment Concentration <br />Sediment Concentrations Increase Substantially During <br />Short Periods of High Streamflow <br />Large sediment concentrations occur during periods of high streamflow caused by <br />intense thunderstorms. Sediment -laden peak flows resulting from these storms <br />usually occursuddenly with rapid increases in water- surface elevation <br />(or stage). <br />Sediment concentrations of 50,000 to 200,000 <br />milligrams per liter during peak flows are common in <br />areas underlain by the Raton or Poison Canyon <br />Formation, which consist primarily of sandstone and <br />shale. A sediment -laden peak flow is shown below <br />(fig. 6.3.1 -1) at the Sarcillo Canyon streamflow - <br />gaging station (station 11 in fig. 6.3.1 -2), a tributary <br />of the Purgatoire River. The water discharge when <br />the photograph was taken was approximately 11,000 <br />cubic feet per second, and the sediment concentra- <br />tion was about 100,000 milligrams per liter. After <br />the flood peak passed, the stage declined 7 feet in 2 <br />hours. Base flow at this station is 0.06 to 0.20 cubic <br />foot per second, and the volume of sediment carried <br />during base flow is minimal. A typical relationship <br />of sediment concentration to streamflow and to river <br />stage in the area is illustrated in figure 6.3.1 -3 for a <br />56 <br />peak flow at the Carpios Canyon streamflow- gaging <br />station (station 16 in fig. 6.3.1 -2), a smaller tributary <br />to the Purgatoire River. Sediment concentration <br />increased rapidly with streamflow to a maximum just <br />before or coincident with the peak stream discharge. <br />Sediment concentrations during peak flows are <br />much less near headwater regions where streams <br />drain mostly Pennsylvanian and Permian limestone, <br />sandstone, and shale as well as Precambrian igneous <br />and metamorphic rocks. The maximum sediment <br />concentration determined during peak flow in the <br />Middle Fork of the Purgatoire River at streamflow - <br />gaging station 9 (fig. 6.3.1 -2) was 4,000 milligrams <br />per liter. <br />