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Show PART III CURRENT WATERSHED CONDITIONS AND PROBLEMS, AND EFFECTS OF PROBLEMS The Upper Colorado River drainage basin embraces 109,580 square miles of land. This excludes the 3,916 square mile Great Divide closed basin which is included in the Upper Colorado Region for this study. The average annual sediment yield (years 1914-1957 adjusted to 1957 condition) of this basin at "Lee Ferry", its discharge point, was .58 acre-feet per square mile per year. See Table 3 for suspended sediment discharge at selected stations for various periods. This yield has historically varied considerably from year to year, but on the basis of period averages it is apparent that a significant change in the average rate occurred in the early 1940's. At Lee's Ferry on the Colorado, for the years 1930-42 average yield was .77 acre-feet per square mile per year, while for the years 1943-52 it was down to .45 acre-feet per square mile per year, and for 1953-62 it was .32 acre-feet per square mile per year. The apparent low rate for the period 1953-62 was due largely to low precipitation for most of the period and partly to deposition in the many recently constructed sediment catchment basins and other structures. The reduction from the 1930-42 period to the 1943-52 period was the result of changes in the factors which affect sediment yield. Specifically, vegetal cover improved and land use and management favored reduction of erosion. Similar reductions are apparent at all points in the region for which long-term sediment records are available indicating that the changes in controlling factors have been general throughout the region. Of the total average annual sediment yield at Lee Ferry for the years 1914-1957 about 20 percent was derived from the Main Stem Sub- region, 27 percent was derived from the Green River Subregion and 53 percent was derived from the San Juan-Colorado Subregion (38 percent from the San Juan drainage alone). Current records as of 1962 indicate that these approximate percentages are applicable at least through 1962. Actual delivery of sediment to Lee Ferry was reduced considerably sub- sequent to 1962 with the completion of several major reservoirs. Sediment yield problem The past, present, and projected sediment yield situation is shown in the figure on the following page. The sketch of past yields represents the generalized conditions which prevailed at the time watershed manage- ment programs and certain land use controls were initiated. This did not occur simultaneously throughout the region. It began around the turn of the century and spread to most ownerships over the next 30 to 35 years. 36 |