Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name

Environmental Studies

Department or School/College

Environmental Studies Program

Committee Chair

Vicki Watson

Commitee Members

Don Potts, Scott Woods


Suspended sediment in forested streams, Water quality in Blackfoot Basin in Montana


University of Montana


This thesis assesses efforts to reduce sediment impacts in Cottonwood Creek, a tributary to the Blackfoot River in western Montana. The first objective evaluated trade-offs in stream crossing improvements regarding short-term sediment impacts versus long-term reductions in sediment load from road surface erosion and possible culvert failures. Suspended sediment and turbidity measurements were taken during spring snowmelt the year before and after a culvert replacement by a bridge, and during the replacement activity. The two study years were typical snowmelt years; i.e., 2- and 4-year return intervals, based on a ten-year USGS period of record. Culvert fill and road surface erosion measurements were also taken. Likely sediment load from upgrading a culvert was compared to that of not upgrading a culvert. Upgrading probably produces less sediment over the long-term than not upgrading. The second objective assessed other stream crossings in high-risk areas in the same watershed to determine culvert failure risk and to estimate how much sediment load could be produced from culvert failures and road surface erosion. The annual sediment yield from culverts predicted to fail within 20 years and from estimated road surface erosion modeled over ten years was much lower compared with the literature, even for undisturbed forests. Two hypothetical scenarios were compared—in one, culverts that were expected to fail were replaced with bridges; in the other, they were not replaced and did fail. Replacing the culverts with bridges resulted in a six percent increase in sediment load to Cottonwood Creek, but this amount of difference is likely within the error range of these estimates. Hence there seems to be little long term benefit in replacing the culverts. The third objective critiqued the TMDL/Water Quality Improvement Plan for the Middle Blackfoot/Nevada Creek basins. While the TMDL involved considerable detail on sediment sources, quantities, and proposed reductions in loads, the implementation and monitoring features were weak. Based on examining sediment reduction efforts in these three ways, this thesis concluded that stream crossing improvements, such as replacing culverts with bridges, are likely to reduce watershed sediment loading over the long-term despite short-term disturbances by these efforts.



© Copyright 2009 Natalie Regina Marie Shapiro