Didymosphenia geminata, (Lyngbye) M. Schmidt 1899, is a stalk forming colonial diatom that is exhibiting increased nuisance and invasive behavior in running and standing freshwater systems, particularly in regulated streams the Western United States. D. geminata blooms are a nuisance because of their persistent stalk material which composes ~90% of the colony. The stalk material forms a mesh, filtering and sequestering particulates in the periphyton mat altering the benthos. Invertebrate biodiversity is dependent upon nutrient cycles and trophic energy flow which makes changes in community composition and taxon densities useful in understanding environmental changes (Vannote 1980). The overarching hypothesis is the degree of D. geminata coverage in Boulder Creek, a regulated stream, alters the benthic macroinvertebrate community by altering community composition, total invertebrate density, functional feeding guild composition and size of macroinvertebrates. Secondarily the mats are expected to be mainly inorganic material with the abundance of D. geminata cells increasing as coverage increases. The study season was the summer of 2008. All sampled sites were in the Boulder Creek Watershed along the Front Range of the Rocky Mountains in Northern Colorado, U.S.A. Four hydraulically similar sites with varying degrees of D. geminata coverage were chosen between two stream gauges. Sites were divided into cross-sections and sampled for water chemistry, periphyton and macroinvertebrates. The sampling periods were separated into three distinct flow regimes for data analysis: pre-peak flow, post-peak flow and low-flow. There was an increase in D. geminata cell abundance as D. geminata coverge increased. There was also a significant amount of inorganic material within the mat samples. The three hydrological flow regimes showed no significant differences in the periphtyon or macroinverterbate communities. The general macroinvertebrate density was not affected by the degree of D. geminata coverage. The small Baetidae and Chironomidae macroinvertebrates, collectors and scrapers were favored in the higher impacted D. geminata sites. Whereas, small Lepidostomatidae macroinvertebrates and shredders were favored in the low impacted D. geminata site. These results, however, could not be significantly correlated to D. geminata coverage. The lack of significant relationships could be attributed to the lack of variation in D. geminata coverage over the sampling period.