How floods are predicted

Forecasters use past experience, computer models, knowledge of flood-prone areas, weather forecasts, and observed data such as rainfall, river stages, and water flow to make predictions about how high rivers and streams will get at certain locations and when they will recede.

Knowledge of topography, geology, and hydrology is important. The following are two distinct ways that the basin's topography and geology contribute to flooding.

The first situation occurs when a section of river is very wide, but then is suddenly squeezed into a steep, narrow gorge. During heavy rainfall events or when the winter ice begins to break up, the increased flow of water or ice backs up in the narrow gorge, causing the river to overflow its banks. Also, when the ice jam breaks, a sudden surge of water can cause downstream flooding.

The second situation occurs when a river flows through an area with very little slope, and shallow banks. In this topography, which is fairly common in the basin, the river levels out and flows slowly. During heavy rainfall events, the river quickly swells and overflows its banks. When winter ice breaks up, the slow-moving flow causes the ice to jam easily, creating obstacles and backing up the water.