The Alberta Rivers App displays water elevation levels and the calculated flow rate at recording stations across Alberta. How is the flow rate calculated, what is the accuracy of the recording, and how do they compare to other recording on the same watercourse?
Bottom line: The flow/discharge rates are accurate when comparing data between years at the same site. The flow rates are determined using a gauge height-discharge relationship which is continually being adjusted to take into account constantly changing stream channels. Flow rates (discharge) are determined via ratings curves (gauge height vs Q) that are developed by comparing measurements of gauge height (stage) to measured discharge.
Stream gauges continuously measure water height (stage). This continuous record of stage is translated to river discharge by applying the stage-discharge relation (called a rating curve). Stage-discharge rating curves are developed for stream gauges by physically measuring the flow of the river regularly and at a wide range of stages; for each measurement of discharge there is a corresponding measurement of stage. Special effort is made to measure extremely high and low stages and flows to capture the true shape of the curve. The stage-discharge rating curve depends upon the shape, size, slope, and roughness of the channel at the stream gauge and is different for every stream gauge.
The development of an accurate stage-discharge relation requires numerous discharge measurements at all ranges of stage and streamflow. In addition, these relations are continually checked against on-going discharge measurements because stream channels are constantly changing (erosion or deposition of streambed materials, seasonal vegetation growth, debris, or ice. New discharge measurements plotted on an existing stage-discharge relation graph would show this, and the rating curves are continually adjusted to allow the correct discharge to be estimated for the measured stage.
If anyone wants to delve into the topic, they can look at the Water Survey of Canada stage-discharge manual:
https://publications.gc.ca/collections/collection_2021/eccc/en37/En37-464-2016-eng.pdf
