Graduation Date

Fall 2020

Document Type

Project

Program

Master of Science degree with a major in Environmental Systems, option Environmental Resources Engineering

Committee Chair Name

Eileen Cashman

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Brad Finney

Second Committee Member Affiliation

HSU Faculty or Staff

Third Committee Member Name

Margaret Lang

Third Committee Member Affiliation

HSU Faculty or Staff

Subject Categories

Environmental Resources Engineering

Abstract

Salmon Creek watershed is located in the Headwaters Forest Reserve in northern California and is known for the ecological value of its old-growth redwood forest, high biodiversity, and sensitive habitat for endangered species. The Bureau of Land Management primarily manages the Reserve. The land-use history of the Upper Salmon Creek watershed includes extensive timber harvest and road development. Watershed restoration in the Upper Salmon Creek watershed started in 2000 with the primary goal for the Reserve to protect and recover ecologic diversity and threatened native species. Since then, of the 23 miles, 13.5 miles of logging roads and 101 stream crossings have been decommissioned and treated, with 2 miles maintained, 2.4 miles passively restored, and 5.1 miles still requiring assessment. The restoration work is focused on long term reduction in sediment delivery from erosional sites that have historically degraded water quality in the Salmon Creek watershed.

A stream monitoring station is located in the Upper Salmon Creek watershed that uses a turbidity threshold sampling protocol based on turbidity, stage, and temperature. The objective is to evaluate the data collected from Water Year (WY) 2012 to 2019 to assess the sediment yield in the watershed. The field and laboratory data collected at the stream monitoring station were used to further understand the relationships between hydrology, sediment transport, and land-use, and to estimate sediment load from WY 2012 to 2019. Additionally, two precipitation-monitoring stations were installed in the Upper Salmon Creek watershed during WY 2019, to provide a more spatially representative rainfall data set.

Discharge rating curves and turbidity-sediment rating curves were developed to estimate continuous discharge and suspended sediment concentration (SSC) at the stream monitoring station, which were then used to estimate annual sediment yield. The annual sediment yield from WY 2012 to 2019 ranged from 9 tons/mi2 to 178 tons/mi2 (49 tons to 944 tons). The discharge rating curves need to be established every year due to periodic geomorphic changes in the channel cross-section at the monitoring station. However, the relationship between turbidity and SSC does not appear to change from year to year, and the existing data that was collected is adequate to quantify the relationship between turbidity and SSC. The exception is at high values of turbidity, where it is recommended to reprogram the automated sampler to draw water samples only at the very highest turbidity thresholds to refine the relationship with SSC.

On average, storms were responsible for 77% of the total annual sediment yield over the period of record. Annual sediment yield varied from year to year, and seemed to be strongly influenced by the characteristics of individual storms within a WY such as rainfall intensity in the watershed. Additionally, episodic events such as bank failures and landslides may have elevated turbidity and discharge. Estimating sediment load by individual storm events is the first step in exploring the meteorological, hydrological, and other temporal changes that contribute to variability in sediment transport and sediment load. Future studies should focus on normalizing the data to effectively remove the interannual variability in sediment yield that is related to rainfall variability and episodic events so that trends in sediment yield that may be due to land use changes and restoration actions can be identified.

Citation Style

APA

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