Over time, a number of flow measurements are done at different water levels and a relationship between flow and water level is developed. This “rating curve” or “stage – discharge (water level - flow) table” allows the observer to estimate flow from any measured water level. This is a cheap and fast way of getting reliable streamflow data. Therefore, the streamflow gauges are located in places where the rating curve remains relatively stable over time. At each of these locations, the river is geomorphologically and hydraulically stable meaning it does not change significantly with time, exhibits an unique relationship between water level and flow (such as at bridge sites or water control structures), and is not affected by downstream conditions.
The manual measurement of water level can be done by placing a measuring device or “staff gauge” in the river attached to a bridge or somewhere so that the gauge is always in the water and can be easily read by an observer. Usually, the staff gauge elevation is related to the geodetic datum by surveying from an established geodetic benchmark. This allows comparison among gauge sites.
Manual measurement of water level can be collected in an economical way and, provided there is a local observer during extreme events, they can be very useful in documenting rare events and calibrating numerical models. For long term data, electronic continuous recording units are used. A sensor reads the water levels usually at one hour intervals and that data is recorded in a “data-logger” that can be downloaded at the site or by telephone, radio or satellite communication. |
Rideau River at Ottawa