Beaver, as allogenic ecosystem engineers, are capable of extensively altering surrounding habitats resulting in significant effects on stream water quality. This paper presents insight into downstream effects of beaver impoundments (both actively maintained and abandoned by beaver) and accumulated sediments on water quality, especially in protected watersheds serving as a drinking water supply. In a tributary un-inhabited by beaver compared to those with active beaver ponds, we found significantly (p < 0Ð001) lower concentrations of NO 3 -N (respectively: mean (standard error)) without beaver 0Ð11 (0Ð005) mg/l and with beaver 0Ð14 (0Ð002) mg/l), NH 4 -N (0Ð007 (0Ð002) and 0Ð24 (0Ð002) mg/l), dissolved organic carbon (DOC) (3Ð47 (0Ð081) and 8Ð37 (0Ð203) mg/l), chlorophyll (2Ð38 (0Ð066) and 6Ð96 (0Ð203) µg/l), and temperature (16Ð43 (0Ð101) and 19Ð85 (0Ð123)°C), but higher pH (7Ð17 (0Ð02) and 6Ð72 (0Ð022)), PO 4 (0Ð20 (0Ð01) and 0Ð17 (0Ð014) mg/l) concentration, and dissolved oxygen (DO) saturation (100Ð8 (0Ð708) and 67Ð6 (1Ð74)%). Within the beaver ponds, we observed a high abundance of predatory zooplankton, which help to control algae and feed on detritus within the pond ecosystem. We intentionally breached an abandoned beaver dam in the field and conducted a sediment resuspension experiment in the laboratory to assess the potential for increases in turbidity and nutrients from beaver pond sediments as would be expected in the event of a catastrophic dam breach. We found that abandoned beaver ponds have a stronger influence on downstream water quality than actively maintained ponds owing to higher turbidity with finer sediments and sharply increased stream water temperature during a dam failure.