Understanding the ecological processes that impact parasite abundance and distribution is critically important for epidemiology and predicting how infectious disease dynamics may respond to future disturbance. Digenean trematodes (Platyhelminthes: Trematoda) are parasitic flatworms with complex, multi-host life cycles that include snail first-intermediate hosts and vertebrate definitive hosts. Trematodes cause numerous diseases of humans (e.g. schistosomiasis) and livestock (e.g. fascioliasis), and impact the ecology of wildlife systems. Identifying the ecological mechanisms that regulate these complex, multi-host interactions will advance both our understanding of parasitism and the dynamics of infectious disease. By examining patterns of infection in Elimia (= Oxytrema = Goniobasis) proxima snails, my dissertation research investigated the environmental factors and ecological processes that structure trematode communities in streams. First, I examined temporal variation in trematode infection of snails in five headwater streams. Over a three year period, I found no consistent seasonal patterns of trematode infection. There was consistency across sites in trematode prevalence, as sites with high prevalence at the beginning of the study tended to remain sites of high infection, relative to lower prevalence sites. Second, I examined landscape level variation in trematode infection by characterizing the regional distribution, abundance and diversity of E. proxima infections in 20 headwater streams. I found a broad scale spatial pattern in trematode communities due to regional turnover in dominant species. This pattern was correlated with elevation, but there were no significant relationships with other environmental variables. Additionally, molecular characterization of trematodes indicated the presence of cryptic (morphologically indistinguishable) species complexes within this system, and variation in genetic diversity among trematode types may reflect differences in host dispersal abilities. Third, I examined trematode infection within a single stream network across multiple headwaters and the mainstem. I found a decreasing downstream gradient of trematode prevalence related to several environmental variables including elevation, snail density, conductivity, and stream depth. Additionally, headwater communities were nested subsets of the communities found in the mainstem. By combining approaches at different temporal and spatial scales, my dissertation research increases our understanding of the processes that impact the abundance and distribution of parasites.