Spring-run Chinook salmon, Oncorhynchus tshawytscha, of the Feather River in California is a threatened species of economic, cultural, environmental, and recreational importance whose wild population is rapidly declining. A major source of juvenile mortality is caused by Ceratonova shasta, a myxosporean parasite with a complex life cycle and two mandatory hosts: salmonids, and the definitive freshwater annelid worm Manayunkia occidentalis. Salmonids and C. shasta have been extensively studied in this river, yet the locations of the annelids are unknown. Locating them is crucial for informing hatchery Chinook release strategies and water management to support outmigrating juvenile Chinook. Therefore, this study focusses on studying the annelid host. An infectious zone has been identified downstream of the Thermalito Afterbay Outlet. We hypothesize that the highest population density of M. occidentalis can be found in the previously identified infectious zone due to nutritional content originating from the Outlet. To investigate this claim, we will conduct a field study, water nutrient analysis using spectrophotometry, and determine prevalence of C. shasta infection in annelids through qPCR. Data will be analyzed using PCA to identify which environmental factors are most likely to explain the density of the annelid populations and prevalence of infections with C. shasta.