Date of Award:

2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Advisor/Chair:

Patrick Belmont

Abstract

We analyzed recent total suspended solids (TSS) data from 45 gages on 36 rivers throughout the state of Minnesota. Watersheds range from 32 to 14,600 km2 and represent a variety of distinct settings in terms of topography, land cover, and geologic history. Our study rivers exhibited three distinct patterns in the relationship between discharge and TSS: simple power functions, threshold power functions, and peaked or negative power functions. Differentiating rising and falling limb samples, we generated sediment rating curves (SRC) of form TSS = aQb, Q being normalized discharge. Rating parameters a and b describe the vertical offset and steepness of the relationships. We also used the fitted SRCs to estimate TSS values at low flows and to quantify event-scale hysteresis.

In addition to quantifying the watershed-average topographic, climatic/hydrologic, geologic, soil and land cover conditions, we used high-resolution lidar topography data to characterize the near-channel environment upstream of gages. We used Random Forest statistical models to analyze the relationship between basin and channel features and the rating parameters. The models enabled us to identify morphometric variables that provided the greatest explanatory power and examine the direction, form, and strength of the partial dependence of the response variables on individual predictor variables. The models explained between 43% and 60% of the variance in the rating curve parameters and determined that Q-TSS relation steepness (exponent) was most related to near-channel morphological characteristics including near-channel local relief, channel gradient, and proportion of lakes along the channel network. Land use within the watershed explained most variation in the vertical offset (coefficient) of the SRCs and in TSS concentrations at low flows.

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