ABSTRACT
In 1977 the La Mesa fire burned forested portions of the Jemez Mountains and surrounding volcanic plateau in northcentral New Mexico. Fluvial systems in the devegetated watersheds are adjusting to different sediment and water discharge conditions. Instruments have been placed in six small watersheds to measure these adjustments in areas of different burn intensity (a qualitative estimate of devegetation). The amount of sediment delivered to the stream systems is influenced by: 1) the amount of hillslope devegetation; 2) seasonal variations in weathering and runoff; 3) protective post-fire forest litter; and 4) sediment production from burrowing animals. Sediment source areas in devegetated watersheds offer little resistance to erosional processes, and, as a result, threshold conditions are frequently attained. The primary source areas of individual watersheds are the basin headwaters and distal portions. Mid-basin regions of the watersheds studied underwent very little erosion during the period of measurement. Overland flow distances are greater in the mid-basin regions, requiring longer time periods to remove hillslope detritus. Extensive rilling in the basin headwaters and channel incision near basin mouths are post-fire fluvial adjustments. Cut and fill cycles in channel alluvium of the higher-order streams are related to seasonal variations in sediment supply. Accelerated mechanical weathering during freeze-thaw cycles and concomitant accelerated erosion increase the magnitude of sediment delivery to the streams in extensively devegetated areas. In areas of less devegetation, variables such as post-fire vegetal litter (needlecast) reduce rainsplash erosion and sheetwash erosion; however, accelerated sediment production by burrowing animals yields unexpectedly high erosion rates. Systematic adjustments between process and form in undisturbed watersheds of the western United States occur, in part, because geomorphic processes are adjusted to watershed vegetational characteristics. In devegetated watersheds sediment yield is influenced more by accelerated mechanical weathering or post-fire vegetal litter than by drainage basin size or slope.
A process-response model is developed to illustrate the complexities of fluvial adjustments in devegetated, mountainous terrain. This model differs significantly from the complex response model for alluvial valleys. In mountainous areas, tributary streams will adjust independently of trunk drainage lines where bedrock nickpoints separate the two. Thus, tributary streams can be semi-closed systems which supply sediment and water to trunk streams but do not adjust to any modifications made by the trunk streams. Revegetation and erosional stabilization of these semi-closed systems may occur before200 the stabilization of the trunk streams.
