Didymosphenia geminata (Didymo) is a freshwater diatom that is considered a nuisance algae because it produces thick mats of polysaccharide stalks (i.e., blooms) under certain environmental conditions. Didymo blooms may adversely impact aquatic habitat and food webs, especially in regions supporting cold-water fisheries. In 2015, Didymo was first discovered blooming in Michigan waters in the St. Marys River, a Great Lakes connecting channel that supports a major recreational fishery and is nearby numerous small, cold-water streams. Therefore, there is interest in understanding spread, detection, and drivers of blooms in this region. To understand Didymo spread, we completed 50 surveys in Upper Peninsula streams by collecting benthic scrapes for Didymo cell counts and water samples for eDNA analysis. Additionally, we attempted to understand the threshold of eDNA detection using two methods. We sampled downstream of a known Didymo infestation in the St. Marys River, evaluating the effect of distance and volume on eDNA detection, and used artificial streams with varying concentrations of Didymo to assess the effect of density on detection. Finally, we used artificial streams to study how nutrients may contribute to Didymo blooms. We found Didymo spread throughout the 112-km channel of the St. Marys River and reached peak densities (up to 40,000 cells/cm²) within two years, however, cells and eDNA were not detected in any nearby streams. Concentrations of Didymo eDNA were low and did not differ with distance from the known source and was not detected in the artificial streams with low Didymo densities. Experimental nutrient additions are underway and will provide improved understanding of ecological drivers of Didymo blooms. Due to the spread throughout the St. Marys River and Lower Michigan, continued research of detection methods and drivers of blooms is needed to understand the threat of Didymo to cold-water Michigan rivers.