Laserfiche WebLink
Cross section #4 is located upstream of the gravel pit approximately 0,6 <br />_ miles, and is not within the portion of the stream channel that is <br />currently headcutting and eroding. Cross section #4 indicates that there <br />has been some aggradation of material in this portion of the stream channel <br />(Figure 4), This aggradation may be attributed to incidents such as the <br />headwall failure and mass-wasting from a slope on the east side of the <br />canyon, upstream from the town of Illium. At this site, a landslide <br />covered the Illium Valley road and contributed large amounts of material to <br />the stream channel. Cross section #4 indicates that channel incision and <br />widening are not occurring in this portion of the stream channel, as they <br />are downstream near the gravel operation. <br />Changing the base level on the main channel also affects tributaries to <br />that channel, causing similar channel adjustment processes as the <br />tributaries respond to the new base level, An example of this can be seen <br />in the mainstem San Miguel River, from the confluence area upstream toward <br />the county bridge (Photographs 14 and 15). Headcutting is proceeding <br />upstream on both the mainstem San Miguel and the South Fork, as the streams <br />attempt to adjust to a new base level created by excavation of material <br />below the confluence (photographs 11, 12, and 13). The channel will <br />continue headcutting unless a natural control, such as bedrock, exists <br />within the channel. Heede (1986} notes that where base level changes <br />,. occur, serious consequences may follow for both the stream and the adjacent <br />riparian area. <br />Stream channel downcutting has also caused the water table to drop, which <br />has negatively impacted wetlands and riparian vegetation within the <br />floodplain. Dewatering of wetlands located near the confluence of the <br />South Fork and mainstem San Miguel river has reduced habitat complexity <br />within the aquatic ecosystem. Incised channels intercept and drain <br />existing water tables, which leads to dewatering, alteration and <br />destruction of riparian ecosytems (DeBano and Schmidt, 1989). Johnson <br />(1987), notes that off-site impacts from gravel mining may include <br />hydrological changes and sedimentation and that mining may lower the Water <br />table for the surrounding area, contributing to reduced productivity or <br />vegetation mortality adjacent to the site, Heede (1986) notes that channel <br />incision causes bank instability and lowering of streamside water tables, <br />which endangers riparian ecosystems, <br />According to Norwood Ranger District personnel familiar with the area, <br />riparian vegetation upstream of the confluence and adjacent to the gravel <br />operation has declined rapidly, within the course of 2 to 4 years. Rapid <br />water table decline has been documented as causing the collapse of riparian <br />poplar forests (cottonwoods) in several papers (Rood and Heinze-Milne, <br />1989; Rood and Mahoney, 1990; Mahoney and Rood, 1991, 1992). About 575 <br />cottonwood trees within Mary E campground (located about 300 feet upstream <br />of the confluence) decreased in vigor and leaf output, and, in some cases, <br />the trees died. The cottonwood stand in the Mary E campground is one of <br />the only three large, contiguous stands of cottonwoods in the South Fork <br />drainage. In June, 1993, the Forest Service had about 550 cottonwoods cut <br />