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46 the highest elevation of the riparian zone. This transition line was interpreted as the highest recent water surface elevation. A comparison between the demarcation line of the two zones and the 1983 high water elevation was surveyed in the field. In only three of fourteen surveys, the 1983 high water surface was higher than the riparian line. The 1983 surveys ranged from two foot below to one and a half feet above the riparian zone upper limit. On the average the highest point of the riparian zone was approxi- mately one half foot above the 1983 high water elevation. This close correspondence between the high flow last season and the riparian zone limit implies the riparian zone is inundated by peak flows with a return period of approximately seventeen years or equal to that of the bankfull discharge. A reduction in the magnitude or frequency of flows inundating the riparian zone will result in the encroachment of riparian-vegetation into the channel area. This invasion would result in a loss of flood flow carrying capacity. The active channel is maintained by the peak flows on- an annual basis. Bankfull discharge removes any vegetative invasion in riparian zone. Physical Model Study A physical model of the cobble substrate at river mile 16.5 was constructed in the eight foot flume at the CSU Engineering Research Center hydraulic laboratory to study the physical processes and measure- ment of sand transport over cobble substrate. A detailed description of the model investigation is presented in Appendix A. A discussion of the results follows. The hydraulic and cobble substrate conditions of the channel at River Mile 16.5 were modeled on a one to one model to prototype basis (Photo 2). Three separate tests were performed. The purpose of the first test was to calibrate the sampling efficiency of the Helley-Smith bedload sampler on a cobble bed for a range of hydraulic conditions less than those required for incipient motion of the cobbles. The efficiency of the Helley-Smith was calibrated for hydraulic conditions over a sand bed in the second test. The third test consisted of an investigaton of sand scouring processes in cobble substrate. The data collected during these tests included water discharge, depth, mean velocity, velocity profiles, water surface slope, flume bed slope, Helley-Smith sediment load and total sediment load. From the calibration tests of the Helley-Smith sampler, the following results are reported: 1) The Helley-Smith sampler will underpredict the sand bedload transport over a uniform cobble bed. 2) The sampler overpredicts for a sand bed with large transport rates. 3) Diversity of substrate improves the collection efficiency of the sampler.