Transport of tracer gravels on a coastal California river

Tracer gravels can provide information about the transport of coarse gravels not sampled by standard sampling methods. White dolomite fragments emplaced as riprap on the Carmel River, Monterey County, California, were entrained by progressively higher flows in 1981, 1982, and 1983 and were transported progressively further. Following the 1981 flow, tracer gravel concentration decreased exponentially downstream. After the 1982 flows (and again after the 1983 flows), the distribution of gravels reflected channel morphology, not an exponential downstream decrease. Peak concentrations corresponded to locations of major gravel bars. While tracer concentration decreased overall in the downstream direction, its distribution suggested movement by discrete “jumps” from bar to bar rather than continuous movement. Calculation of bedload transport rates for rocks > 45 mm by spatial integration of tracer data yields bedload transport rates of about 3–5% of the total bedload flux measured by Helley-Smith sampler 5.5 km downstream. The spatial integration method is not strictly applicable here because injection of tracer was not instantaneous and channel form is quite irregular. Nonetheless, it is notable that results by this method are compatible with sampled transport rates downstream. The results of this study show the dominant influence of channel morphology on tracer gravel distribution, reflecting the importance of in-channel storage sites in coarse sediment movement. This effect and the low recovery rate reported for tracer gravels in the literature are reason for caution in using short-term tracer gravel studies to compute rates of coarse bedload flux. However, tracer studies can be profitably used to provide data on the movement of coarse particles unmeasured by Helley-Smith samplers.