Date of Award:

8-2026

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Watershed Sciences

Committee Chair(s)

Charles Hawkins

Committee

Charles Hawkins

Committee

Scott Miller

Committee

Julianne Scamardo

Committee

W. Carl Saunders

Committee

Daren Carlisle

Committee

Christopher Lant

Abstract

Large-scale assessments of stream health (ecological integrity) are required by policies such as the U.S. Clean Water Act and provide critical information needed to properly manage public lands. Our ability to quantify the ecological condition of streams in the western U.S. and identify the causes of degraded conditions has previously been impeded by a lack of standardized large-scale datasets, high natural temporal and spatial variability of ecological attributes, and poor-quality land use data. I compiled large-scale monitoring datasets and built models to predict the values of ecological attributes (metrics) at a given site in the absence of human impacts. In Chapter 2, I show how these models improve the ability of agencies to consistently assess the biological condition of different types of streams, including those that go dry at some point in space or time. In Chapter 3, I show that the same type of models increased the number of physical habitat metrics that are useful for managers. In Chapter 4, I applied existing models and physical habitat models developed in Chapter 3 to assess the conditions of streams on Bureau of Land Management (BLM)-managed lands in the conterminous western U.S from 2013-2015 and 2019-2022. These models, together with better representation of arid regions during surveys and the recent availability of better land use data for BLM-managed lands, allowed me to more comprehensively assess the condition of western streams compared with previous assessments. Biological conditions based on macroinvertebrate assemblages were more extensively degraded than water chemistry and physical habitat conditions but improved between surveys (49% versus 36% of stream length degraded) likely due to cooler temperatures and higher precipitation during 2019-2022. The degradation of macroinvertebrate assemblages was primarily associated with stream drying, higher stream temperatures, higher salinity, higher nitrogen concentrations, higher percent fine sediment, and lower percent bank shade. These instream conditions were associated, in turn, with oil and gas wells, livestock grazing, agriculture, and dams. These results can assist the BLM with prioritizing ecological attributes for management actions. Future research should focus on determining the relative importance of climate versus land use on instream conditions.

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