Figure 3-2: Sequential Degrading of Stream Channel
|USDI-BLM, USDA-Forest Service. 1995. Rangeland Reform '94 Draft Environmental Impact Statement. Bureau of Land Management. Washington, DC: 3-45 (citing USDI-BLM. 1993. Riparian area management: process for assessing proper functioning condition. Tech. Ref. 1737-9. Bureau of Land Management, Service Center. Denver, CO.).|
USDI-BLM, USDA-Forest Service. 1995. Rangeland Reform 94 Draft Environmental Impact Statement. Bureau of Land Management. Washington, DC: 3-44 - 47.
Riparian communities in good condition are fragile and complex. They act like
a huge sponge or natural reservoir in times of water abundance, then, through
capillary action, slowly release stored water during dry periods of the year
(BLM 1989). This results in moderated stream flow yearlong for perennial streams
or extended periods of flow for intermittent streams (Heede 1977; Brinson
and others 1981, Winegar 1977). In some cases, restored riparian habitats will
reestablish perennial flow in streams that are intermittent in a deteriorated
A healthy riparian community protects streambanks from erosion and maintains a high water table and productive habitat for fish and aquatic invertebrates. Overhanging vegetation protects water from direct solar heating and covers fish while they hide and rest (BLM 1989). Healthy riparian communities also provides habitats for hundreds of terrestrial species, significantly contributing to the biological diversity and quality of the ecosystem (Thomas and others 1979).
Excessive livestock grazing [sic] affects many resources watersheds, but no community is more susceptible to degradation than those associated with aquatic resources. Beginning at the headwaters, livestock severely trample source springs and destroy protective riparian vegetation and reducing spring outflow. Without shade from riparian vegetation, solar radiation rapidly increases water temperatures (F&WS and NMFS 1981).
Downstream, livestock heavily concentrate in the riparian zone removing protective vegetation. Trampling results in soil disturbance, particularly in wet meadows and stream channels. Erosion of the stream channel is accelerated, eventually resulting in a lowered water table, reduced water storage capabilities of streambanks and floodplains, and altered streamflow morphology (F&WS and NMFS 1981; Winegar 1977).
Altered streamflow morphology typically increases frequency and intensity of flooding (no retention of precipitation) and reduced late summer flow or loss of perennial flow when water is needed most. Increased runoff or frequent flooding further increases erosion, resulting in widened and straightened stream channels, which allows increased water velocity during flow periods and increased exposure of the water to sunlight. During low flow periods living space for fish is significantly reduced and water temperature elevates rapidly due to increased exposure to solar radiation. In addition, water for use in irrigation and watering of livestock is reduced.
As erosion progresses and water tables lower, natural grass meadows are left high and dry. Once meadow grasses die, brush species, such as sagebrush and rabbitbrush, immediately encroach and reduce the amount and quality of forage (BLM 1993g). Figure 3-2 shows the sequential degrading of a stream channel and its associated riparian community (wet meadow). As riparian resources degrade, accelerated erosion incises stream channels, lowering water tables and restricting historically wide floodplains to narrow riparian communities in wash bottoms. Figure 3-3 shows recovery of stream-associated riparian areas.
Heavy livestock grazing [sic] most severely affects the stream channel. Livestock tend to spend a large portion of their time within the riparian community because of the lush vegetation and shade. As a result, livestock consume a greater percentage of riparian vegetation than they consume on surrounding uplands. While grazing, livestock trample riparian vegetation and streambanks. Eventually protective riparian vegetation is lost. Streambanks are sheared off through trampling and become erodible (Bowers and others 1979).
Once streambanks are broken down and eroded, streams are left wide and shallow with significantly less living space or hiding cover for fish. Wide streams have huge surface areas exposed and susceptible to increased water temperatures and rapid evaporation (Brown and Krygler 1967; Crispin 1981). Eroding streambanks contribute excessive sand and silt accumulation over the stream bottom, decreasing aquatic invertebrates (fish food) production and smothering fish eggs in spawning areas (Armour 1978).
The most significant results of excessive livestock grazing [sic] in riparian areas are as follows:
Figure 3-3: Recover of Stream-Associated Riparian Area
|USDI-BLM, USDA-Forest Service. 1995. Rangeland Reform '94 Draft Environmental Impact Statement. Bureau of Land Management. Washington, DC: 3-46 (citing USDI-BLM. 1993. Riparian area management: process for assessing proper functioning condition. Tech. Ref. 1737-9. Bureau of Land Management, Service Center. Denver, CO.).|