Ice Conditions Warning
Info Notes on Ice Conditions and Data Spikes/Dips
Ice Conditions Make Water Level and Discharge Data Invalid
Rivers/streams in Canada frequently experience ice conditions for part or all of winter months. Users of this website should know that the presence of ice in a stream has an effect on water level and on the relationship of water level and flow (stage-discharge relationship).
For a given streamflow, the presence of ice generally causes a rise in water level. This is because there is more resistance to water flow and resulting lower velocity of flow. That means that a larger cross section of flow is needed to pass the given rate of streamflow. This is the case even with a relatively small amount of ice at the streambanks, and certainly so with larger amounts of ice. Even if there is no ice at the location of a hydrometric station, the presence of ice downstream can have a similar effect, by causing the water to "back up" in the channel - this is called backwater and can also happen because of other barriers to flow downstream of a hydrometric station.
Another thing for the website user to note is that water levels may be inaccurate for other reasons (problems with water level sensing systems resulting from frozen conditions), so further to the general description above, the water level shown on the website may not be the actual ice/water level at the gauge.
The other impact of ice conditions in the vicinity of a hydrometric station is that the stage-discharge relationship is made invalid. As mentioned above, the water level is higher for a given flow than would be the case for open water conditions. The discharge data shown on the real-time hydrometric website (e.g. discharge graphs) are based on the stage-discharge curve for open water conditions. Because the water levels at the station are raised by ice conditions, the discharge computed from the open water stage-discharge relationship is invalid. In general the true discharge is less than that calculated by the stage-discharge curve. But if the water level is in error because of sensing system problems, the discharge would be totally invalid.
As indicated in the site disclaimer, all data on the real-time website is preliminary and subject to errors. It is up to the user to use their own judgment as to the validity of the data, in this case whether the water level and/or discharge shown is reasonable to use for their immediate purpose.
Real-time Hydrometric Data Spikes or Dips
Sometimes the water level and discharge data on the real-time website will exhibit extreme spikes (or dips). These can occur for a variety of reasons, generally related to the electronic functions of data sensors and loggers. The effect of a spike is often that the vertical scale of the graph (the y-axis) is automatically adjusted to show the full range of data, including the spike. That makes the normal range of data appear to be a nearly flat line.
To "eliminate" the spike, the website user can adjust the Y-Axis scale on the graph by inserting values for the minimum and maximum Y-axis values in the spaces on the left below the graph. Using the recent range of water level data as a guide, the user can generally set these values to include the "normal" water level trace, and the spike will extend off the graph. Note that the user may need to extend the time scale back to an earlier date to observe recent normal water levels.
As indicated in the site disclaimer, all data on the real-time website is preliminary and subject to errors. It is up to the user to use their own judgment as to the validity of the data, in this case whether a large shift in water level reflects what is happening at the hydrometric station or is a case of spurious data.
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