Measuring the FloodNumerous factors affect stream flow, and the potential for flooding. The most important factors are the amount and type of precipitation, the nature and condition of the drainage basin, and climate. These factors can cause flooding individually or in combination.

All rivers are subject to fluctuations in flow. During a rainstorm, the amount, intensity, duration, area of storm, and path of the storm, all influence the runoff reaching the stream. The amount, intensity, and duration of storms affect the ability of the land to absorb the precipitation and therefore affect the rate of runoff. The area and path of the storm relate to the area of the basin receiving rainfall, which in turn, represents the area contributing runoff. The area and the runoff rate determine the volume of water that will pass a given point downstream.

1. Amount and Type of Precipitation

Snowmelt runoff - During the Canadian winter, mo

st precipitation is stored as snow or ice on the ground. During the spring, large quantities of water are released as spring melt, causing heavy runoff and flooding. This is called freshet. Snowmelt runoff floods are the most common ty

pe of flooding in Canada.

Flash floods - can be extremely dangerous. Unanticipated, they usually occur in small watersheds as a result of a torrential downpour, often caused by heavy thunderstorm activity. A flash flood occurs when there is extremely heavy rainfall within a short period of time. Flood conditions develop rapidly because the ground is incapable of absorbing the water quickly enough, resulting in a very high runoff rate.

Ice Jams - are a major cause of flooding in Canada. In fact, for most Canadian rivers, the annual peak water levels are due to ice jams. These jams result from the accumulation of ice fragments that build up to restrict the flow of water. JRideau River 2008ams form during both the freeze-up and breakup periods, but it is usually the breakup jams that have the greater flooding potential.

Outburst Floods - are common in western Canada. Lakes dammed by glaciers or moraines suddenly drain and tonnes of water, mud and debris are released. An outburst flood typically occurs when the water level becomes high enough to actually float the ice or when a small channel forms under the ice and causes rapid melting, thus expanding the channel.

Coastal Storms - Many Canadians living along the shores of major lakes, such as the Great Lakes, or along Canada's coasts have experienced flooding and property damage as a result of high wind and wave action, or the interaction between high estuarine flows and tides. Shoreline flooding may be caused by storm surge or seiches, often occurring simultaneously with high waves.

A storm surge or seiche is an abnormal rise in sea level accompanying a hurricane or other intense storm. This is a natural process generated when wind blows in a constant direction and piles water up on a downwind shore. When the wind drops, the water is released and flows back to the opposite shore.

The appearance of a storm surge can be simulated by tilting a basin partially filled with water and then setting it down on a level surface. The water will slosh back and forth at a period determined by the size and shape of the basin.

1. The Nature and Condition of the Basin

The shape, size, soil type, and topography of the drainage basin are other factors affecting the quantity of water reaching the stream. These factors are usually constant. However, the absorptive properties of the soil vary with vegetation cover, season, and previous rainfall.

2. Climate

Climate has an important influence on the relationship between precipitation and runoff. Frost makes most soil impenetrable if the soil contains moisture. In northern latitudes, the winter season - during which a large part of the year's precipitation is stored in the form of snow, is often followed by sudden melting, with much of the runoff flowing quickly over frozen ground to reach streams. Climate controls the strength of an ice cover in winter, and its manner of breakup, which governs the severity of ice jamming.

Adapted from Environment Canada's Freshwater website <