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fires. At higher elevations the changes in forest composition and density are not as apparent, primarily because the <br />duration of the fire suppression period is short relative to the natural fire recurrence interval. <br />9. The changes in forest density and cover over the last century or so are generally believed to have decreased annual <br />water yields. Annual water yields from the 1.34 million acres of national forest lands in the North Platte River <br />basin are estimated to have decreased by approximately 11 to 13 percent or 150,000 to 190,000 acre -feet per year, <br />depending on the assumed stand history for the spruce -fir forests. Hydrologic models indicate that average annual <br />water yields could be increased in the North Platte River basin by about 55,000 acre -feet per year if all 502,000 <br />acres designated as suitable for timber harvest was regularly cut on a sustained yield basis. Similar data are not <br />available for other river basins in Colorado, although the overall trends are probably similar. <br />10. The rates of timber harvest on national forest lands in Colorado substantially increased in the early 1960s, peaked <br />in the late 1960s and late 1980s, and then dropped by about two - thirds in the 1990s relative to peak harvest levels. <br />The projected increase in annual water yields due to timber harvest on national forest lands was nearly 100,000 <br />acre -feet in the older forest management plans, but is projected at 30,000 to 40,000 acre -ft per year in the most <br />recent forest management plans. Recent harvest levels have averaged slightly more than half of the values in the <br />selected alternative in the most recent forest plans. Fuels treatments are projected to increase average annual water <br />yields by 18,000 acre - feet/yr for the eight national forests that estimated these values in their forest management <br />plans, and most forests met their acreage targets for fuels treatments from 1997 to 2000. Because the average an- <br />nual runoff from national forest lands in Colorado is greater than 10 million acre -feet per year, the projected water <br />yield increases from forest harvest and fuels treatments on national forest lands represent less than I% of the total <br />water yield. <br />11. The quality of the water flowing from forested areas is generally very high. Forest harvest and fuels treatments <br />should have minimal adverse effects on water quality if they are carefully designed and conducted in accordance <br />with best management practices. Wildfires pose the biggest threat to water quality and site productivity. In severe- <br />ly -burned areas peak runoff rates can increase by a factor of 10 of more, while erosion rates can increase by 100 <br />times relative to unburned areas. These large increases are due to both the lack of cover and the development of a <br />water - repellent layer at or just below the soil surface. Burning at moderate and low severities causes much smaller <br />increases in the size of peak flows and erosion rates. Data from the Front Range suggest that there is little recovery <br />by the second summer after burning, but the higher runoff and erosion rates should decline to near - background <br />levels by 3 -4 years after burning. Percent bare soil appears to be the dominant control on post -fire erosion rates, <br />so treatments that immediately increase the amount of ground cover are most likely to be effective in reducing <br />post -fire erosion rates. <br />12. This report does not attempt to address the myriad of other issues that must be considered when evaluating various <br />management alternatives for forested lands. Some of these issues include the numerous laws and regulations that <br />affect land management, economic considerations, the downstream uses of water and water storage capacities, <br />and the effects of forest management on recreation, local communities, aesthetics, and other plant and animal <br />species. <br />iv <br />