Definitions from the North American Lake Management Society (NALMS):

Hydrograph - A graphic representation or plot of the flow of water or the elevation of water plotted against time. A graph showing stage, flow, velocity, or other hydraulic properties of water with respect to time for a particular point on a stream. Hydrographs of wells show the changes in water levels during the period of observation.

We use hydrographs in to analyse flows in rivers, levels of lakes and changes in groundwater elevations. Hydrographs can be used to:

• Compare a number of years of data with each other;
• Compare one period against another period;
• Analyze a single event such as a flood;
• Compare different months or seasons;
• Compare different rivers or lakes;
• Compare different aquifers or groundwater zones.

A hydrograph is a good way to quickly analyze the data for a river, lake or well. It puts the data in a visual format that is easy to relate to and gives an context to compare or relate the data to other events or happenings that may be related to the river, lake or groundwater well.

Hydrographs can be plots of different durations of data. As the levels in rivers, lakes and aquifers change constantly (sometimes too slow to see) the values plotted in the hydrograph have to be averaged into meaningful time periods. Common periods are; Hourly, Daily, Monthly, and Annually. The chart below illustrates the difference between monthly and daily averaged data. As you can see, there is much more detail in the daily data when compared to the monthly averaged data. The different time periods of the data allow for different levels and types of analysis to be done with the data.

Hydrographs can also be used to quickly look at a lot of data and compare different time periods. The following long-term hydrograph for the Red Deer River at Red Deer has a couple of interesting points in it. The first is the lack of data during the mid 1930s. This missing period is common to many stations in Alberta, and is the result of decreased monitoring during the Great Depression. The second point of interest begins around 1983. After this point, the minimum flows are higher than for the previous period of record. This is a result of the building and operation of the Dickson Dam. The operations of the dam release more water in the winter than what flowed before its construction.

Hydrographs can also be used to analyze floods. As you can see above, the peak flows differ from year to year. Hydrologists can look at the flows in a river and determine how big and how often floods are likely to occur. For current flood or water quantity information go to the Alberta Environment Water Page. For historical lake level and streamflow data go to the Water Survey of Canada Data Web Page.