Water Levels and Flows

LEVELS
Water level is the height of the water surface, in metres, above an established datum plane where the stage is zero. The zero level is arbitrary, but is often close to the streambed.

FLOWS
Streamflow, or discharge, is the volume of water flowing past a fixed point in a fixed period of time. For water flow in rivers or streams, the value is recorded in cubic metres per second (m3/s).

FLOW REGIME

The flow regime is the range of flows and their seasonality and frequency of occurrence.

Within a watershed or subwatershed flow regime, there are four flows that impact the shape of the channel and valley as well as the ecology of the system:

• Valley/Floodplain flows which are high magnitude flooding events that have a frequency varying from 25 to 100 years.
• Riparian flows which are high flows that access parts of the valley which are dry for most of the time (for example every spring and/or fall).
• Bankfull or channel defining flow which is the maximum flow within a channel before spilling outside the main channel, and
• Baseflow which has the lowest flow on an annual basis supplied from groundwater discharge and usually occurs during the summer months

Factors causing changes to Levels and Flows

• Changes in the water cycle (precipitation, snowmelt, evaporation amounts)
• Wind
• Earth's movements
• Climate change
• Human factors like dredging and diversions

Measuring Levels and Flows
To measure water levels and flow, gauge readers or automatic recording devices can be used. The flow is calculated from several measurements of flow velocity at several depths and points across stream cross-section. These velocities averaged over the cross-section give the flow at that point for that water level. With enough measurements of flow over the same point in the stream for different water levels, a water level-flow relationship is established. The flow rate can then be estimated from future water levels measurements.

The Ministry of Natural Resources Surface Water Monitoring Centre monitors water flow in rivers and streams as well as water levels in lakes and reservoirs to predict and minimize the potential impacts of floods and drought.