In the lower parts of the basin, deposits may create deltas at the river's mouth. The volume of water flowing in a river together with the speed and timing of the flows determines how a river shapes the surrounding landscape and how people can use its waters. Rainfall, snowmelt, and groundwater all contribute to the volume of flow, producing variations from season to season and year to year. In Canada, most high flows are caused by snowmelt and occur in spring. This is the season when floods are most likely to occur.
Rainstorms can also cause high flows and floods, especially on small streams.
The lowest flows on rivers in Canada generally occur in late summer, when precipitation is low and evaporation along with consumption by plants is high, and in late winter, when rivers are ice covered and the precipitation is stored until spring in the form of ice and snow. Lakes Canada probably has more lake area than any other country in the world, with 565 lakes larger than 100 square kilometres. Typical river flows are listed in Figure 5. Water levels are read manually by gauge readers or continuously recorded either digitally or on graph paper. Measurements can be made from a bridge, by wading in a stream, by boat or from a cableway strung across the river.