Jeff Green | Jun 30, 2005
Feature article, July 7, 2005
Feature article July 7, 2005LAND O' LAKES NewsWeb Home
Contact UsLevelling the Waters
by Gray Merriam
All water is not level. Paddlers on fast, steep rivers commonly see the water sloping away downstream. All flowing water is lower at one end -- the oceans -- than at the other. Many of us are 500 feet or more above sea level.
Long shallow lakes can have the water surface much higher at one end than at the other end --particularly when the wind creates an 'internal seiche' -- an oscillation of water from end to end over hours or days. In Lake Erie, one end can be more than a metre higher than the other. Onshore wind can also pile water much higher on one shore than on the other.
But when lakeside or riverside dwellers speak of "levelling out the water", that's not what they are talking about. They are talking about keeping the water at one fixed level for most of the year -- at least during the times when they want to put in their dock and their pontoon boat.
Before control structures (dams), water levels varied a lot. The Ottawa River at Ottawa, in spring, increased to six times its autumn flow rate before humans put dams on all its tributaries. After we controlled the Ottawa, it fluctuated less than half as much.
Variation in water levels is increasing now that climatic forces are becoming much more variable. Although it is difficult to predict exactly where climate changes will be felt, or the exact degree of change, there is widespread agreement that climatic variation is increasing. The relative constancy of the climate of the last several decades is being replaced by unpredictable and large variations in many natural forces. Distribution of rainfall, both in time and in space, is one of those increasing variables. How many multi-inch one-day rain storms can you remember in the last year? April, September, June --. How many were extreme in one region and insignificant not far away?
When one of those multi-inch-in-one-day rains hits the watershed that drains into your lake, you will likely notice and, if not prepared, be inconvenienced. For example, Kennebec Lake, north of Highway 7 has a watershed of about 66,560 acres. The surface of the lake itself is about 1216 acres. For every one inch of rain falling on that watershed, if it all flowed quickly into Kennebec Lake, the surface of the water would rise over 55 inches -- over 4.5 feet.
The salvation is that the watersheds of many lakes are still well vegetated and not much is covered by impermeable roofs and driveways. A lot of the rainfall soaks into the ground or is soaked up by the living and dead vegetation. In addition, there still are many wetlands that catch rainfall that is running off and hold the water like the cells of a giant sponge. Water retained by a lake's watershed is released slowly, often over several months, and that prevents any cresting of the flow into the lake. In the case of Kennebec Lake, The Kennebec Wetland Complex is just such an area of many wetlands that acts as a water-storage mechanism for the landscape and the lake downstream.
But there are a lot of bare rock areas, and roads with graded ditches to hurry the runoff and a significant fraction of each rainfall does flow off each watershed and into each lake. More, if the lake-dwellers don't care for their lake's watershed. For example, in late June, 4 inches of rain fell on Kennebec Lake. A shore-dweller reported that the lake level rose 14 inches. Assume that the whole watershed received the same amount of rain. If all that water ran off into the lake within a few hours, the lake surface would have risen 219 inches, not 14. In fact, only 6.7 percent of the rain falling on the watershed ran quickly into the lake to raise the water level.
High water levels and accompanying high flow rates are the power behind some important natural processes. Spates of high water carry nutrient-rich silts onto flooded areas, fertilizing them and supporting exceptional productivity of shoreline areas. Where high water flows over underwater ledges or obstacles, the current is directed downward resulting in upwelling of nutrients from the bottom into the top-water. Resultant high productivity enriches both the upwelling site and areas downstream. When the dam was built on the Manicouagan creating the big horseshoe-shaped reservoir in the Manic crater on Quebec's north shore, upwelling where the Manic poured into the St. Lawrence was stopped. Instead of periodic highwater crests only steady flows came down the river. What had been a favourite feeding ground for Beluga calves in that upwelling area stopped providing rich food for the calves and they gathered there no longer. Fluctuating water levels are natural and important to some ecological processes, even if they inconvenience some humans and some nesting loons.
What about unvarying water levels that permit docks to be set at one height and never adjusted and permit carefree docking of giant pontoon boats at those fixed docks. Constant water level is unnatural. It is an amenity for shoreline dwellers. But that amenity has the cost of damage to natural processes.
Some of our watersheds, such as the Mississippi have hardly any lakes that are not levelled by dams. Lakes without dams are special and should be highly valued. Certainly more highly valued than some lakes whose depth and surface area and shoreline (and mudflats) are all controlled by edicts from afar or by local disputes.
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