Near-complete destruction of vegetation over 125 km2 near Sudbury, Ontario has increased denudation rates by two orders of magnitude and caused substantial changes in hydrologic regime. Denudation by channeled and unchanneled flow, measured with erosion pins on small plots (2–1000 m2) and a small drainage basin (0.09 km2), averaged 6000 m3/km2 (maximum 24 700 m3/km2) during summer and fall in 1971 and 1972. Maximum denudation occurred during late August to October. Snowmelt runoff in 1972 yielded 1000 m3/km2 of sediment. The weighted average denudation rate, including rates of bedrock disintegration (60–170 m3/km2/y; mean 120 m3/km2/y) is 3700 m3/km2/y.Runoff coefficients average 0.88 for events with return periods between 2 and 10 years; 25% of the May–October rainfall runs off as Hortonian overland flow. Estimated sedimentation rates for three flood-control structures indicate 25% storage depletion over a 50 year period; the return period of floods then able to be retained is reduced to 50 years, compared to the design parameters of 100 year 6 h rainfall (smaller structures) and 100–200 year 12 h rainfall, 6 h P.M.P. (largest structure).