Spring 2021

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Applied Environmental Science


Freshwater systems are a limited resource and must be managed to maintain or restore their ecological health. Bioassessments, which use the biota at a site to draw conclusions on a system’s ecological health, are commonly applied to freshwater systems. Freshwater bioassessment programs are typically only applicable in perennial systems (those which have surface water continually) and do not yet have an accepted role in assessing non-perennial streams in their dry phase. Although dry phase non-perennial streams that have a significant hydrological nexus with traditionally navigable waters navigable waters are protected under the state of California and national legislations, they cannot be assessed with bioassessment currently.

We sampled 106 dry streams in the arid southwestern United States and developed indices of taxonomic completeness (i.e., Observed to Expected or O/E indices) in dry streams to assess the effect of anthropogenic stress at these sites. We did this for three key assemblages in dry streams: channel-dwelling arthropods, riparian vegetation-dwelling arthropods, and bryophytes. We also explored different definitions ways of identifying reference sites and parameters related to index development to assess their effects on O/E indices’ performance. O/E indices using channel arthropods were the most responsive and sensitive to human activity, regardless of which reference definition or probability of capture threshold was used. Channel arthropods were the most responsive likely because they respond in a predictable way to stress. Vegetation-dwelling arthropods and bryophytes were absent at up to one quarter of all sites, which negatively affected index performance. The absence of these assemblages at reference sites yielded less responsive and unsuccessful indices developed with these assemblages.

The ecological status of dry streams can be determined from indices of taxonomic completeness when using channel-dwelling arthropods. Other taxa may not occur consistently enough across sites to produce responsive indices. Vegetation-dwelling arthropods should be omitted from future O/E studies because of their lack of response to stressors in stream channels. Further research should be conducted on moss and their response to local stressors because of their response to stress is difficult to predict.