Why timing is key for sampling water quality in rivers

This article was originally written for and distributed in the September edition of the Bow River Basin Council (BRBC) Preserving Our Lifeline newsletter. I recommend checking it out, as it contains a number of great articles and information! Header photo credit: Christina Suzanne

In large rivers, such as the Bow, there are a wide range of flow conditions that change on a seasonal timescale. Thus, when planning to measure water quality, timing is an important consideration. Ideally, sampling for water quality would be done year-round but due to various constraints, it is predominantly carried out during the late summer and fall months, such as September. Various flow conditions result in very different water quality and as such it is important to consider what you want to measure and if there are any limitations (ie safety or equipment) to your sampling regime. Ultimately, the timing of sampling directly relates to the results you will obtain and the ability for multi-year comparisons.


The hydrology of a river is the driving factor in aquatic ecosystems with flow environments directly influencing both living and non-living components. The Bow is a nival river, meaning that flow is predominantly influenced by snow-melt, with lowest (base) flows occurring while the river is ice-covered and biological activity is slower. Consequently, largest flows generally occur in the early summer when rates of snow-melt are at their highest and there is correspondingly large amounts of summer rainfall. These high flows result in what is arguably the most dynamic time for river ecosystems with substantial amounts of materials entering the river; however, it is also the hardest period to obtain samples.

Sediment and other materials are always present in some amount within the Bow River, however there are distinguishable episodes throughout the year when they peak. During the spring, as the river ice breaks up, it scrapes the banks and river bottom. At the same time, materials deposited throughout the winter on top of snow, begin to enter the river due to melt. The result is high amounts of sediments and other materials being present in the river during this time creating a cloudy or muddy appearance, which ultimately influences the ecology of the river. Sediments can also transport materials from the landscape that alter water quality.

Many of the materials that are deposited on winter snow cover, and ultimately delivered to the river during snow-melt, can be harmful to aquatic organisms in high enough concentrations. While the high flow volumes help to dilute any potential contaminants, it still has direct impacts on organisms in the river. In contrast, the highest concentrations of problem materials often occurs during the late summer as there is less water volume diluting these substances. This is another reason why late summer and fall sampling is so valuable for measuring water quality. The variety of substances which can be considered contaminants at high levels means that measuring them can be quite tricky. One way of measuring the impact these materials have on river health is by using organisms that are low on the food web and seeing how they respond to river conditions.

In our lab, we use specific aquatic organisms, primarily invertebrates (animals without a backbone) to measure water quality. There are several reasons why we use invertebrates, including the fact that they are diverse, a key part of the food web, and highly abundant in freshwater systems. They also are not overly mobile and have relatively long lifecycles which makes them easier to obtain while sampling. The aquatic part of their lifecycle generally occurs in the late summer and early fall and as a result, our primary sampling takes place during this period. Late summer and fall sampling also allows us to collect the invertebrates when they are large enough for easier identification. It also provides us with the easiest and safest access to sample collection sites. Related to the hydrology of the river, this timeframe is also the most stable for the organisms, allowing us to collect them with higher success rates. Invertebrate tissues and community diversity can give us a wealth of information on the chemistry of the river and overall ecosystem health.

In summary, continuous monitoring of river water quality is very important to ensure the integrity of the ecosystem is maintained with rivers being a vital component of surrounding communities. To be able to properly monitor river data over multiple years, water quality samples (ie sediments, contaminants, invertebrates) must be collected in the same season. For example, comparing spring and fall samples from different years will not be consistent, given the information above. The optimal sampling season may vary depending on location and hydrology of the river basin, therefore understanding the background characteristics of the watershed you are sampling is necessary before you begin collecting river water quality data.


For more information about what the Aquatic Ecology Lab does at the University of Calgary, please contact david.barrett@ucalgary.ca or christina.suzanne2@ucalgary.ca.

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