The diversity of beaver dams and pond morphology has been noted by many studies in North America.  In addition, beavers are widely recognized as “ecosystem engineers”, which alter their surrounding environment in different ways. However, little research has identified factors that explain variation in the downstream effects of beaver modifications.  Groundwater hydrology and the associated size and shape of beaver impoundments are potential mechanisms explaining the diversity of impacts of beavers on downstream reaches.  We measured groundwater flow through multiple beaver impacted systems and compared reaches upstream and downstream of beaver ponds in watersheds surrounding the Rocky Mountain Biological Lab in western Colorado. Our survey revealed a variety of downstream impacts related to pond morphology.  Variables such as water temperature, benthic invertebrate phenology, nutrient concentrations (nitrate, ammonium, soluble reactive phosphorous, total phosphorus and total nitrogen), benthic organic matter (ash free dry mass and chlorophyll a content), periphyton biomass and benthic invertebrate community composition showed variable patterns related to groundwater flow and pond morphology. Results of this survey emphasize the importance of understanding how variation among beaver impoundments determines the nature of effects in tailwaters.