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<br />porest-Pire Devegetation and Basin Adjustments <br /> <br />Bedrock outcrops are not extensive in the drainage basins; rather. varying <br />thicknesses of colluvium, water-worked colluvium, and alluvium Cover the <br />basins. Grain-size characteristics for the streams and hillslopes of the water- <br />sheds have been analyzed. by sieve analysis. Grain-size distributions for the <br />intense-burn watershed are summarized in figure 4. Commonly. grain-Size <br />distributions near the hil1s10pe divides are unimodal, whereas a bimodal dis- <br />tribution is typical for downslope sections (fig. 4). Changes in the distri- <br />butions suggest reworking and sorting of the hillslope sediment with a deple- <br />tion in the 2 to 3 + size (sand). Grain-size distributions of the channel <br />alluvium show increases in the 1 to 3 + size downchanne1. This suggests <br />transport of sand grains from the hillslopes to the channels. <br /> <br />MEASUREMENTS <br /> <br />Hillslope sections and channel reaches are instrumented in each basin to <br />monitor the amount. distribution, and seasonal variations in sediment contribu- <br />tion to the stream channel. Modifications of channel morphology are measured <br />with erosion chains (table 2), and the amount of runoff was estimated by <br />slope-area techniques. To measure the hillslope sediment contribution. two <br />types of instruments are employed: (1) erosion pins. and (2) a movable- <br />contour-plotting frame which permits 25 elevation measureme!1ts over a 1 m2 <br />area (Campbell. 1974) (table 2). Validation of erosion measurements deter- <br />mined from the erosion pins and movable-contour-plotting frame is accomplished <br /> <br /> .. HSP I z. HSP 2 .. HSP 3 <br /> 20 zo <br /> " .. <br /> 10 10 <br />>- <br />0 . . <br />z <br />"' <br />:::J -, 0 , z . . >. -, 0 , z -, 0 1 Z . ,>4 <br />0 3 4 )4 <br />"' <br />0: <br />U. <br /> .0 CIA .0 CoB '"' CoC '"' Cho <br />f- <br />Z <br />"' .. z, z. <br />0 <br />0: <br />"' zo <br />11. <br /> <br /> <br /> <br /> <br /> <br />_I 0 1 2: 3; 4 >4 <br /> <br />-I 0 I 2: :5 4-"4 <br /> <br />.1 0 I 2::S 4 >4 <br /> <br />-I 0 1 2: :s 4"4 <br /> <br />GRAIN SIZE IN PHI INTERVALS <br /> <br />Figure 4. Grain-size distributions of hillslope (HSP) and channel (ChJ samples <br />for the intense-burn watershed, Burnt Mesa. HSP 1 is at hillcrest <br />divide and HSP 3 is at bp,se of slope; eHA is at headwater and CHD <br />is near basin mouth. <br /> <br />on. <br />