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W. D. White and S. G. Wells channels aggraded immediately after the fire in response to summer precipita- tion events. Channel widening occurred as point bars and channel bars formed. Some channels doubled in width during this initial response. Small alluvial fans formed at the junctions of low-order drainage lines. Field observations indicated that sediment transport was' dominantly bed- load. Between September and November 1977, high-order channels carried less bedload because coarse sediment supplies from the hillslopes declined. In response to decreasing sediment supply and lower magnitude runoff events. the channels began scouring. , Rejuvenation in the sediment supplies from the hillslopes occurred during freeze-thaw cycles in fall, 1977. The stream channels were able to transport this second sediment pulse without significant change in channel width; however, channel scour did continue (fig. 10). Fine-grained sediment pro- duced by the second pulse was transported as suspended load. Little chan- nel widening is necessary for transporting high silt-;.c1ay loads (Schumm, 1960) . Incision of most streams during this period is related to frost heaving in the channel bottoms. Moisture in the alluvium repeatedly froze and up- lifted fine material which could then be easily 'transported. This process accounts for increased channel incision during! freeze-thaw cycles (fig. 10). Following these two sediment pulses, the winter snowpack prevented ex- tensive erosion (table 4) and stabilized channels. During the spring 1978 melt and runoff, channel incision recommenced and continued until mid-summer, .... E ~2.5 Z o (J) 20 U' wZ >- ~~ <tZ 1,5 ..JZ =><t ::EJ:' =>U 1.0 UlJ.. o J: " I- 0,5 0.. W o o INTENSE BURN WATERSHED BURNT MESA . - OCt. NO'L 1977 Dee. Feb. May June 1978 July Jon. Mar. Apr. TIME (MONTHS) Figure 10. Rate of channel incision at 'the mouth of the intense-burn water- shed, Burnt Mesa. 216