The U.S. Geological Survey is investigating sources of fecal bacteria in streams in the Grand Valley of western Colorado by using quantitative polymerase chain reaction microbial source tracking (MST) markers. The CowM2 MST marker was not detected in water-quality samples collected in 2022, which raised questions about the use of the marker for investigations in western Colorado. The presence or absence of bacteria associated with MST markers can be affected by geographic location, animal health, and diet. Known-source cattle fecal samples were collected in the Grand Valley to determine the presence or absence of the CowM2 marker in the feces of local cattle. A total of five samples were collected, representing herds grazing on public lands and local farms.