D--048966 - CALFED Bay-Delta Program - State of California

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899 Hydrology and Water Resources sensitive to logging-related disturbance within 30 m of the site in Arizona. Since then, debate has continued about the channel (Erman and Mahoney 1983) and perhaps within 100 validity of similar statements applied to watersheds throughm (McGurk and Fong 1995). In a recent study of forest man- out the West. The impacts of grazing that relate to hydrology agement effects on aquatic habitat in the Sierra Nevada, data depend primarily on the behavior of the animals: feeding, were collected on channel characteristics, aquatic habitat, fish drinking, producing waste, and traveling. If the animals reabundance and health, aquatic invertebrate abundance, large main in one place too long and consume much more than woody debris, water chemistry, and management history in about half the available forage, vegetative recovery may be twenty-eight different basins in the Sierra Nevada (Hawkins impaired and an excessive amount of bare soil may be exet al. 1994). In general, natural variability in the measured posed to erosive rainfall (Fleischner 1994; Committee on attributes masked effects of management activity. Response Rangeland Classification 1994). Although the amount of conof aquatic organisms to disturbance in the watersheds tended sumption that constitutes "overgrazing" depends on vegetato be small compared with response to natural factors. Ob- tion and site characteristics (Menke et al. 1996), half of the served increases in nutrient loading and temperature ap- initial forage is a useful, though admittedly crude, rule of peared to enhance abundance of some taxa without any thumb (California Division of Forestry 1972). When insuffinoticeable adverse impacts on others. Most of the communi- cient vegetation remains after grazing, raindrop impact can ties in the observed streams appeared to be limited by food change surface conditions and consequently reduce infiltraresources. The study noted, "We cannot at this time either tion and increase erosion (Ellison 1945). Soil can become commeasure or predict with any degree of reliability or confidence pacted by the repeated pressure of moving animals, especially the cumulative effects most types of land use practices will if the soil is wet. The combination of soil exposure and cornhave on natural ecosystems" (Hawkins et al. 1994, 1). paction can decrease infiltration and increase surface runoff. If infiltration capacity is severely limited on a large fraction of a catchment, the extra runoff can quickly enter streams and generate higher peak flows (e.g., Davis 1977). G R A Z l N G Surface and Channel Erosion Grazing of domestic livestock has probably affected more area Exposure of mineral soil and enhanced overland flow also in the Sierra Nevada than any other management practice accelerate erosion. A variety of studies around the West have (Menke et al. 1996). Over the past century and a half, cattle found dramatic increases in sheet erosion and gullying in and sheep have been virtually everywhere in the mountain overgrazed sites compared with ungrazed areas (Fleischner range that provides forage. The near-ubiquitou s presence of 1994). Severe gully erosion in the uplands of the North Fork--~ grazing animals has left few reference sites that we can be Feather River has been caused by decades of overgrazing|¯ certain were never used by livestock. The best approxima- (Soil Conservation Service 1989). Nevertheless, the worst ero- ’\ tions to ungrazed conditions are those areas that have been sion problems associated with grazing typically occur near rested for a few decades. Even Sequoia Nationa! Park was streams. Cattle tend to congregate in riparian areas for obvigrazed until 1930 (Dilsaver and Tweed 1990). The absence of ous reasons: abundant food, water, shade, and lower temperareference sites leaves us uncertain about what an ungrazed tures. Consequently, riparian vegetation is overgrazed, banks stream looks like and how it functions. This uncertainty is are trampled and eroded back, and bed deposits are disturbed. not merely an academic concern. Major questions of grazing All this activity adds significant amounts of sediment directly management depend on our confidence in our understand- to the stream. Dislocation of sediments in the streambed by ing of how natural systems function without human-induced moving animals augments suspended sediment. Degradation perturbations. For example, we can hypothesize that over- of riparian vegetation permits bank erosion to accelerate ungrazing on the Kern Plateau in the 1800s contributed to the der the more frequent peak flows that are caused by the dewidespread arroyo development and conversion of wet mead- crease in infiltration capacity. About half of the channels in ows to dry terraces. There are several lines of evidence that the Meiss allotment in the Upper Truckee River watershed support that hypothesis. However, we would have more con- were identified as being in fair or poor condition as a result fidence if one of the early shepherds had invested a couple of of overgrazing (Lake Tahoe Basin Management Unit 1993). summers in fencing off an entire watershed and preventing Changes in channel morphology have been related to overentry just to satisfy the curiosity of future generations. This grazing in headwater streams tributary to the Carson River problem of uncertainty exists to some degree with all impacts, (Overton et al. 1994). Elimination of riparian vegetation by but there are many areas that were not mined, dammed, overgrazing in the broad alluvial valleys of the North Fork logged, roaded, or urbanized. There just are not many that Feather River has led to rapid channel widening and massive were not grazed, sediment loads (Hughes 1934; Soil Conservation Service 1989). In 1924, Aldo Leopold wrote, "Grazing is the prime factor In other areas, such as meadows of the Kern Plateau and San in destroying watershed values," in reference to an overgrazed Joaquin River Basin, downcutting has followed overgrazing D--049011 D-049011
900 VOLUME II, CHAPTER 30 (e.g., Hagberg 1995). Development of these deep arroyos has lowered the local ground-water table and transformed wet Improved Practices meadows into dry terraces supporting sagebrush. The possi- Improved grazing practices as applied to the Sierra Nevada bly compensatory effects of less bank storage and less tran- have the potential for limiting many of the possible adverse spiration by vegetation determine whether low flows in impacts (Albin-Smith and Raguse 1984). Major changes in summer are decreased or increased by the downcutting. A grazing practices in some parts of the eastern Sierra Nevada recent study of channel characteristics between pairs of cur- have recently occurred. A study is currently under way monirently grazed areas on national forests and long-rested areas toting the response of macroinvertebrates and fish to riparin national parks in the Sierra Nevada found significant dif- Jan fencing and rest-rotation management (Herbst and Knapp ferences in bank angle, unstable banks, bed particle size, and 1995a, 1995b). Degraded channels in the Meiss allotment at pool frequency (U.S. Forest Service 1995b). Significant differ- the south end of the Lake Tahoe Basin led to a decision by the ences in undercut and unstable banks were also observed Forest Service to rest the allotment for five to fifteen years between grazed areas and adjacent fenced exclosures with a until stream-bank vegetation has recovered (Lake Tahoe Bafew years of rest. sin Management Unit 1993). However, this decision was overturned on appeal by the regional forester in 1995. Water Temperature Removal of riparian vegetation and channel widening by grazing expose the stream to much more sunlight. Therefore, stream temperatures in summer may be several degrees higher than if shade remained. In winter, the absence of riparian vegetation may allow wind scour of snow in exposed U R B A N, S U B U R B A N, A N O E X U R B A N D E V E L O P M E N T creeks in high-elevation meadows. With less snow serving as The population of the Sierra Nevada foothills is expected to insulation, ice formation may be greater than in creeks with increase rapidly in the next few decades (see Duane 1996a). vegetation capable of trapping more snow, which provides Conversion of forests and woodlands to residential and corninsulation itself. These artificial changes in temperature im- mercial land uses has several serious hydrologic effects on pact aquatic organisms that rely on a more natural tempera- local streams (Lull and Sopper 1969). Such conversions drature regime, matically alter the disposition of rainfall or snowmelt on the landscape by reducing infiltration capacity of the surface to Water Pollution zero or near zero. Land that was formerly well vegetated and rarely, if ever, produced overland flow is converted to an im- Congregation of cattle in and around streams provides a di- pervious zone where virtually all precipitation becomes imrect pathway for nutrients and pathogens to degrade water mediate runoff. The extent of such changes and the ability of quality (Springer and Gifford 1980; Kunkle 1970). High nu- adjacent land to absorb the additional runoff determines the trient loads promote the growth of aquatic algae, which can response of streams. Gutters, ditches, drains, channels, culvirtually clog streams at low flow. An example of prolifera- verts, and storm sewers are intended and designed to convey tion of aquatic plants apparently augmented by cattle is found runoff as rapidly as possible to streams. The combination of in the Owens River above the Benton Crossing road. High greater volume of runoff, faster generation of runoff, and levels of coliform and other bacteria have been found in greater channel efficiency moves more water downstream streams heavily used by livestock (Lake Tahoe Basin Man- faster than under natural conditions. This convergence of large agement Unit 1993; Central Valley Regional Water Quality volumes of water in short periods of time produces frequent Control Board 1995). Cattle grazing in backcountry areas pro- floods downstream from even modest rainfall (Leopold 1968). vides a source of Giardia cysts (Suk et al. 1985). The more frequent floods lead to channel enlargement by erosional processes (Dunne and Leopold 1978; Booth 1990). Un- Associated Impacts fortunately, stream gauging stations have not been placed in strategic locations to actually record changes in stream-flow There are a variety of ancillary effects of grazing. Road construction to provide access to range improvements has simi- regimes in response to development in the Sierra Nevada. lar impacts to those of roads in general, depending on the location and design. Springs are extensively developed for Impervious Surfaces stock watering, at the expense of native biota. Irrigated pas- The proportion of impervious area created by residential conture consumes immense quantities of water for a low-value struction is a rapidly decreasing function of lot size. Small product (Romm et al. 1988). urban lots can be effectively sealed over three-quarters of their surface area, while lots of 0.4 ha (1 acre) might be impervious on only 10%-15% of the area. So-called low-density residen- D--04901 2 D-049012
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D--048966 - CALFED Bay-Delta Program - State of California

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