An Initial Measure of Suspended Sediment Flux from Big Creek and its Tributaries, Central Idaho

This project provides a first exploration into the relationship between suspended sediment concentrations and water discharge within Big Creek as its tributaries. The results from our research provide insight into two fundamental questions: (1) Are the concentrations of sediment observed in tributaries and mainstem streams proportional to the size of their drainage basin? (2) Do sediment concentrations increase systematically with discharge do concentrations fall off earlier than flood flows? During the summer of 2008, water samples were collected from Big Creek at the Taylor Ranch Bridge as well as from 11 tributaries to Big Creek (Cliff Creek, Dunce Creek, Goat Creek, Pioneer Creek, Cave Creek, Cabin Creek, Cow Creek, Canyon Creek, Monumental Creek, Cougar Creek, and Rush Creek). Water samples were collected using three different techniques: (1) an automated pump sampler that made frequent collections from Big Creek, (2) a tethered, hand-held isokinetic sampler that collected depth and width integrated samples off the Taylor Ranch bridge, and (3) a rod-mounted isokinetic sampler that was used in tributaries where the streams could be safely waded across. Our results suggest that larger streams carry disproportionately higher sediment loads than the smaller streams. A suitable explanation for this observation is that as drainage basins increase in size, the probability of a high sediment supply point source (such as a landslide or weak lithologic unit) increases. For example, Big Creek carries more sediment than other channels because it includes tributaries such as Monumental or Crooked Creeks that have characteristically higher sediment loads. Over a three month sampling period the average sediment concentration for Big Creek was .916 mg/L, as compared to a smaller tributary, such as Cliff Creek that has a sediment concentration value of .445 mg/L (difference in drainage area between the two stated streams is 1425.34 km2). Our results also suggest that sediment fluxes remain high during flood events such as spring runoff or summer thunderstorms. In most landscapes, the sediment concentration increases during the rising limb of the flood, but all readily available material is typically exhausted before the high flows decrease. In Big Creek and many of the tributaries, the sediment supply appears inexhaustible. This could be due to high sediment yields from the extensively burned landscape, or from the mining of sediment from abundant floodplain cut banks. Further work can strengthen these findings by concentrating on a smaller number of tributary streams so that a higher temporal frequency of data can be collected. This will be useful in measuring stream health, water quality, and the effect on riparian and aquatic ecosystems. Also, future work should examine the how the proportion of organic sediment to inorganic sediment varies both in time and from one tributary to the next. Though our study aimed to explore how fire influences sediment production, there too many uncertainties in these mechanisms to confidently attribute differences flux to fire history