In response to a congressional request, GAO evaluated streambank erosion problems on the Upper Missouri River, focusing on: (1) whether the Army Corps of Engineers conducted any erosion study before planning and constructing its six dams and lakes on the river; (2) the extent of erosion problems that are caused by the Corps' operations; (3) the environmental and economic impacts of erosion; and (4) the Corps' legal authority and liability related to the erosion problem. GAO found that: (1) the Corps reasonably did not evaluate streambank erosion problems during the planning stages of the dams because it was not required to do so; (2) while the Corps' dams have decreased erosion, they have also eliminated offsetting soil accretion; (3) in 1987, the Corps identified 192 potential erosion control sites on the Upper Missouri and estimated that irrigation would cost $103.6 million; (4) the few environmental impact statements that the Corps prepared for Upper Missouri projects noted only minor environmental effects; (5) the cost of erosion control structures in North Dakota would greatly exceed the economic benefits; (6) the Corps has statutory authority to construct erosion control structures only when they are economically justifiable; and (7) while property owners might claim damages for bank erosion, they would have little chance of winning compensation.