Dr. MacDonald specializes in how land use and vegetation changes affect runoff, erosion, and sediment yields, particularly in forested areas. His initial focus was on runoff processes, hydrologic change, and wetland hydrology, but he has increasingly focused on erosion issues because humans can have a much greater impact on erosion and sedimentation rates. He is widely known for his work on the hydrologic and geomorphic effects of fires, roads, and timber harvest. He also is particularly interested in spatial scale issues and cumulative watershed effects. Current projects include: measuring and predicting post-fire erosion and sediment delivery; studies on road erosion, road decommissioning, fire-road interactions, and road-stream connectivity; sediment budgets and timber harvest effects in northwestern California; and directing a long-term curriculum development project in Vietnam.