Some of the biological effects were investigated of a proposed Corps of Engineers' freshwater diversion to the Boston (Mass) water supply from the Merrimack River. The project examined nearshore fisheries resources including yellowtail flounder, winter skate and land dab. Detrital discharge by the river is of probable importance in the nearshore benthic food web supporting large populations of yellowtail flounder and other benthic fishes. A survey of anadromous fish passage facilities on the river revealed badly deteriorated ladders at the Lawrence and Lowell dams. The proposed diversion of water may reduce attraction capabilities for returning shad, Atlantic salmon and alewives and there by reduce probable successful reintroductions and ladder restorations.