Impacts of Cattle Grazing on Local Riparian Wetlands
Sheep herders formerly movedtheir sheep from one feeding place to another, despite heat, and mainly confined their movements and grazing to upland regions. Sheep were often "bedded down" near water and "favorite sites"were often employed. However, day time grazing near streams or riparian wetlands was generally short.
Most cattle use on public lands occurs during the hot season when cattle seek cooler areas whileforaging, such as riparian wetlands. Therefore, riparian wetland sedges and other riparian wetland vegetation frequently become over grazed. Reduced height of sedges and other species comprising riparian wetlands not only reduces replenishment rates for organic material, but it has other adverse effects as well. Wildlife frequently suffers when riparian wetlands are over grazed, e.g., reduced cover for sage hen chicks and less food for wildlife. However, rapid losses are very important of organic matter and decreases in snow and water retention directly related to overgrazing by cattle.
More local riparian wetlands receive moisture from winter storms than from rainfall. Thegeneral precipitation pattern for the high mountain desert area is early spring rain, followed by little precipitation during the summer, additional rain in the fall, and then early winter and late winter snow storm events. Almost continuous winds occur throughout the cold desert area during all seasons of the year. Winter winds affect snow retention rates of riparian wetlands, and upland areas. Winds at all seasons affect evaporation and/or transpiration rates of riparian wetlands, streams, lakes and ponds, as well as surrounding uplands.
Retention of snow during windy periods is directly related to the size of openings in an object in the windís path, density of cover or size of openings, cross sectional areas affected, obstacle height, fetch, and wind velocity. Fetch is defined as the length of an area serving as a source of blowing snow to a downwind location. The greater the height and cross sectional area of any obstacle, the greater the snow retention.
A formula for a 50% porous snowfence on level terrain is Qc = 6.7H2.2; where Qc is storage capacity in cubic feet of water equivalent per foot. Assume H is height of riparian wetland vegetation in feet. A two-inch height for remaining riparian wetland vegetation provides a Qc value of 0.13, while a 14-inch height yields a Qc value of 9.46 cubic feet of water (adapted from Williams, Mark 2000). A variable not considered in the formula is depth, something a fence does not have, since riparian wetlands can be many feet long. Nevertheless, assuming the relationship between vegetation heights holds true, riparian wetland vegetation 14 inches in height appears to be 72 times more effective at trapping snow than riparian wetland vegetation 2 inches in height. This finding has major implications for riparian wetlands that depend on snowfall for their major water supply.
In the local area, the presence of some thistle infestation on the fringes of riparian wetlands is notuncommon. But, invasion of thistles into the main portion of riparian wetlands is an indicator that riparian wetlands are probably undergoing a drying cycle and are losing, rather than restoring, humus (Figures 6. and 7.). The presence of upland plant species within the wetlands, as documented during PFC investigations by BLM, are also indicators affected riparian wetlands are drying up. Should the water table beneath the riparian wetland surface rise due to increasing humus and humic materials, thistles and upland plant species diminish. On the other hand, as water tables continue to recede from abusive use of riparian wetlands and the loss of humus, thistles will diminish once their roots can no longer obtain sufficient water.
BLM managers frequently base short term cattle grazing limits on a minimum height in inchesfor "remaining stubble." When the minimum stubble height is reached, cattle are rotated to another area. The most important management consideration regarding stubble height of riparian wetlands should not be stubble height but; "Will enough excess riparian wetland vegetation be left at the end of the season to maintain or increase previous accumulations of humus within the riparian wetland, irrespective of stubble height? When snow storms contribute significantly to riparian wetland water supplies, "the more abused the riparian wetland has been in the past, and the smaller the amount of remaining humus, the greater the quantity of vegetation that should remain at the end of the season, no matter what stubble height might be attained."
A white hummock color is perhapsfitting, for only a small percent of the humus or humic material from former times remains to retain moisture. It is as if the riparian wetland has become so desiccated that all that remains is a whitened skeleton of its former self.
These hummocks no longer function as water retention bodies,but respond only to individual precipitation events. The small percentage of moisture they absorb is gone long before the next precipitation event occurs. The riparian wetlands have literally died, and no longer function as water reservoirs (Figures 10. & 11).