Stream velocity is a strong control on both erosion and sediment transport. The size of a stream is most commonly measured by its discharge. Flooding is the normal response of a stream to an unusually high input of water in a short time. Regions at risk from flooding, and the degree of risk, can be identified if accurate maps and records about past floods and their severity are available. However, records may not extend over a long enough period to permit precise predictions about the rare, severe floods. Moreover, human activities may have changed regional runoff patterns or stream characteristics over time, making historical records less useful in forecasting future problems, and long-term climate change adds uncertainty as well. Strategies designed to minimize flood damage include restricting or prohibiting development in floodplains, controlling the kinds of floodplain development, channelization, and the use of retention ponds, levees, and flood-control dams. Unfortunately, many flood-control procedures have drawbacks, one of which may be increased flood hazards elsewhere along the stream. Flash-flood warnings can be an important tool for saving lives, if not property, by making timely evacuations possible.