Title

A Comparison of Sediment Monitoring to Sediment Facies Mapping in the Middle Fork Payette River, Central Idaho

Publication Date

2-2001

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

James McNamara

Abstract

Bedload, suspended sediment, and discharge were monitored to quantify sediment budget for the Lower Middle Fork Payette River. This budget is compared to the sand storage derived from sediment facies mapping of two reaches, one of which is inundated with "clean" sediment caused by sand sedimentation and typical of the present river. Both approaches are used to quantify a total maximum daily load (TMDL) of bedload sand and give similar results, indicating that the sediment facies mapping may be a more efficient way to characterize and quantify sediment loading.

During the study period, bankfull and peak water discharges were primarily attributable to snowmelt. Bankfull discharge at the upper reach is 47.7 m3/s and the lower reach is 55.7 m3/s. Flood frequency analysis at the lower reach revealed the bankfull and peak water discharge observed during this study had return periods of 1.5 and 4.5 years, respectively.

Sediment transported as bedload comprises approximately 80% of the total sediment load within the study reach. The bedload consists of sand (D50 ≈ 1.4 mm) moving over a coarse, armored layer (D50 ≈ 56.0 mm). For water discharges observed during the study period, the armored layer remained immobile with a few isolated exceptions where particles less than 64 mm (b-axis) were mobilized. Bedload transport rates averaged 52 tonnes/day and 162 tonnes/day at the upper and lower reaches, respectively, and the transportation of bedload always occurred at the lower reach. The discrepancy in bedload transport rates was is to the abundance of sand and greater stream power at the lower reach. Because of the discrepancy, the sediment budget calculated a net loss of 43,880 tonnes of sand from the study reach during the 1999 water year.

For the sediment facies mapping, six facies were delineated by the percentage of surficial sand coverage and relative mobility of coarse fraction. Sand volume sampling was conducted using a stratified, systematic sampling grid. The upstream and downstream reaches held 0.061 and 0.189 m3/m2, respectively, or 198 and 687 m3 for equivalent 150 m reaches. The Dune facies, defined by 90 to 100% surficial sand coverage, stored the 85.6 and 97.2% of the upper and lower reaches' sand, respectively.

Presuming that the upper reach has the desired streambed conditions, then the sediment facies mapping indicates that the sand stored in the lower reach needed to be reduced by 76 ± 30%. Sediment monitoring of bedload transport yielded a required 68 ± 16% reduction in supply at the lower reach. These statistically identical results suggest that sediment facies mapping is a viable and cost effective alternative to direct monitoring for characterization and quantification of sand storage. Additional benefits of streambed facies mapping include increased sediment budget resolution, greater knowledge about available fish habitat, and increased predictive bedload transport equation accuracy.

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