Flow Regime

“Water is the driver of Nature.” — Leonardo daVinci

The Rouge River and its tributaries offer a vast array of habitats to plants, animals, invertebrates, and even microbial life. Not only are these organisms important to our local ecosystems but they also provide critical ecological services that benefit human communities more generally (i.e. water purification, flood control), many of which are not visually apparent, and therefore typically undervalued.

Healthy aquatic ecosystems maintain a "dynamic equilibrium”. It is important to understand that when this "dynamic equilibrium" is upset, rapid changes in the aquatic ecosystem can occur. Rivers and streams and their related ecosystems have adapted to their environmental condition over thousands of years, a “climax community”, and cannot respond at the same rate as changes that take place on the landscape. The result is an ecosystem unable to adjust its physical processes fast enough to accommodate what is deemed to be a healthy aquatic ecosystem. The connection between changes in the aquatic ecosystem and what happens more broadly on the landscape may not be overtly obvious, but the connection is water itself.

What is Changing?

A natural flow regime, which is dynamic, has strong influences on the many physiochemical processes in watercourses, which work together to create diverse aquatic habitats. The flow regime is composed of both surface water and groundwater flow and is characterized by the volume of water passing a given point over time. The components of flow regime include magnitude, frequency, duration, timing (seasonality), and rate of change (flashiness) in flows; all play a direct or indirect role in maintaining the ecological integrity of the aquatic system. To illustrate this, the figures below show the Rouge River’s flow regime, both for the main Rouge River and the Little Rouge River. A graph of a highly altered flow regime, Black Creek, an urbanized stream in the Humber River watershed, is also illustrated below. The three graphs collectively illustrate the difference between three flow conditions with the Black Creek being the most highly altered,
the main Rouge River showing signs of significant alteration, and the Little Rouge River showing very limited signs of
flow alteration.

Changes in one or more of the components that make up flow regime, due to landscape modification (increased overland runoff and less infiltration), will result in a change in one or all of the variables that make up aquatic ecological integrity. An important component of the flow regime, particularly in urbanized portions of the watershed, is stormwater flow. Thus, storm water management (and the policies and regulations that govern it) are of particular importance in trying to maintain a natural flow regime and healthy aquatic ecosystems.

Aquatic and riparian species have evolved specific ecological, physical, and behavioural adaptations so that their life histories coincide with, or take advantage of, the “natural” flow regime. This allows some species to breed successfully because they have timed their breeding activities around periods of erosive flow, for example. Changes in the flow regime alter these evolutionary relationships. As such, flow changes may, over time, eliminate native species that are poorly suited to the new ecological condition, and benefit those that are better adapted to the new condition (e.g. generalist and/or invasive species).

It is apparent from the recent watershed plan and the related modeling and analysis reports for the Rouge River watershed, that flow regime plays a major role in shaping the aquatic ecosystem’s form and function. It is not just the quantity of flow that influences the aquatic system, but also the timing and relative distribution of flow that plays a key role. It is now critical that this process be recognized in landscape planning that takes place within the watershed if our rivers and streams are to be protected for future generations.

Stream Gauge Analysis

The two figures below represent an analysis of the Rouge River flow information using the Indicators of Hydrologic Alteration software developed by the Nature Conservancy. The analysis looks at the period before and after 1976 to illustrate how the river’s flow conditions are changing over time. This information allows us to see more clearly what is changing in the river and then relate it to both the rivers geomorphology and its aquatic communities (e.g. fish and benthic invertebrates).

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