A dominant flood type in the Lower Mainland is river flooding (also known as fluvial or riverine flooding). A flood occurs when a river channel reaches capacity and water overflows the riverbank onto adjacent land, called the floodplain.
The Fraser River is the longest river in BC (1,375 km) and drains nearly a quarter of the province. The Fraser flows down from the mountains, through the interior of the province, to the Lower Mainland and out to the Salish Sea (Strait of Georgia). Fraser River flooding is a natural process that shaped the Lower Mainland region over time. It is of concern to communities because of widespread development in the floodplain.
Fraser River flooding is possible during spring freshet (spring snowmelt) when the water volumes increase and the river runs high and fast. Land along the much of the Lower Fraser River is lined with protective earthen barriers called dikes. Dikes help prevent the overflow of water, but in a large-scale flood, there is potential for dikes to be overtopped or to otherwise fail. In a Fraser River flood, it may take days or weeks for flood waters to recede. In some cases, dikes can exacerbate flooding or extend the duration of flooding. For example, in cases where a dike breaches, the remaining intact dike length can cause floodwaters to pond and can delay the drainage of floodwaters from the flooded area.
The volume of flow in the river is determined primarily by the amount of precipitation in the watershed and the rate at which it reaches the Lower Fraser.
In simple terms, freshet flow on the Fraser River is a based on how the following conditions combine in the spring:
- Snow volume (or snowpack) – larger snowpacks in the Fraser River watershed contribute to a larger river flow
- Temperature – warmer weather increases the rate at which snow melts, increasing flow
- Precipitation – rain falling on snow makes the snow melt faster and further adds to flow.
To learn more about snowpack and flood advisories for the Fraser River watershed, visit the River Forecast Centre of BC.
Fraser River flow is generally reported as a flow rate (in cubic metres per second – m³/s) at Hope, where the Canadian Hydrographic Service operates a real-time gauge. Some flooding of riverbank areas along the Lower Fraser is expected when flows at Hope reach 12,000 m³/s. In the largest flood of written record, which happened in 1894, the Fraser River had a flow of 17,000 m³/s, when large areas of the Lower Mainland were flooded: see Flood History.
As flows get higher, larger areas are flooded. Modelling and mapping in the Lower Mainland Flood Management Strategy initiative shows that Fraser River floods are expected to increase in size and severity in the decades ahead under climate change. Learn more.
To find Lower Mainland flood maps, both local and regional, see Flood Maps.