USING THE STREAM-GRADIENT INDEX TO EVALUATE MEADOW FORMATION IN THE SIERRA NEVADA, CALIFORNIA

The stream-gradient index (SL=DH x L/DL) is an equation based on the longitudinal profile of a stream and provides an approximation of stream power (Hack, 1973). The stream gradient (SL) index values derived from the equation are capable of providing insight into the underlying material and geological processes of the watershed being analyzed (Hack, 1973, DeGraff, 1981, Lee and Tsai, 2010). Within the forested areas of the Sierra National Forest, there are a number of meadows; naturally occurring openings where high-groundwater conditions persist. The groundwater supports a herbaceous vegetative community and limits encroachment of trees from the surrounding forest. Wood (1975) demonstrated that many of these meadows are Holocene accumulations of interbedded sands and organic-rich silts. On the landscape, these meadows occupy a variety of locations along the channel within different watersheds. The stream-gradient index was applied to channels traversing eleven large meadows within tributaries of the San Joaquin or Kings rivers to determine if the meadow locations were consistently associated with a distinct range or a threshold SL value. This was accomplished based on data generated from digital 7.5-minute topographic maps. The maps were loaded into Arc GIS so that shape files of the individuals streams could be generated to obtain the data for computing the SL index values along a stream channel. It was expected that the similar climatic conditions and underlying granitic bedrock would result in similar SL values where deposition to form meadows has occurred. The SL values in meadows varied from 36 to 377; values that were commonly lower than those in other reaches of the same channel. However, there was no threshold SL value which was exclusive to meadow reaches among the different watersheds analyzed.