This is part two of our series Where Did All The Walleye Go? It was written by Larry Jones who is part of a group trying to Save The Walleye.
KAWARTHA LAKES-Larry noticed that about 1969 Trent-Severn began a program of dam upgrades. The log dams were replaced, bit by bit, with bottom discharge dams. Larry began to investigate the consequences of this new system. The following deductions from that hypothesis are self-evident. They require nothing more than common sense, together with a rudimentary knowledge of the subject:
1. Moving bodies of water, such as rivers, reach a uniform temperature. This is why we stir a cup of coffee after adding the cream.
2. This uniform temperature rises, and then falls, as we move through the four seasons. Many macroinvertebrates depend on this regular cycle of temperature changes throughout the year. It is the biological clock that controls the various stages of their metamorphoses.
3. The top layer of water upstream of a dam is essentially stagnant. This layer heats up much more that the similar layer in a moving river. The density of water decreases as the temperature rises. This, in turn, creates an abnormally steep thermocline in the water body upstream of dams.
4. These regions of warm, still, nutrient rich water are ideal for the growth of algae. The result is increased turbidity, with ever greater absorption of the heat from the sun. The overnight respiration of suspended algae leads to a decrease in dissolved oxygen concentrations in the upper layer – the epilimnion.
5. The lowest layers of water slide along under this static warm upper layer. Thus the cold, deep, layers are in constant contact with the decaying vegetation on the bottom. Decay uses up oxygen from the water in this lowest level – the hypolimnion. The construction of bottom discharge dams has depleted the oxygen concentrations at both the surface, and deep layers.
6. The annual raging torrents caused by spring melt waters are no more.
This annual event was like a fire hose clearing out a drain. The result is the accumulation of decayed matter, and silt, especially in the marshes. This environment is now suitable for the growth of charra. It can completely take over a marshland; where it reduces the available fish habitat. It also clogs intakes used for irrigation, makes swimming or boating difficult, and reduces the aesthetic appeal of our lakes. The silty bottom is now a problem for native plants such as vallisneria (eelgrass), and elodeas, that need to be firmly anchored. They have difficulty in getting their roots down through the silt into the firm bottom. This new balance of nature is clearly evident in the Bobcaygeon area.
7. The water at the bottom of the waterway is much colder than the surface; by as much as 20C.
When this very cold water pours out below the dam the biothermal clock in (2) is sabotaged. The water in the river is now colder than it should be. Many macroinvertebrates depend on a regular cycle of temperatures throughout the year. When we change that, we compromise their survival. For instance, a stonefly may feel the cold temperatures, and delay its metamorphosis. This may mean that at a critical stage in its life it will be living in the depth of winter rather than in autumn, as it should have been. Fish who live on these stone flies, shad flies, and others, are now starved out of existence.
8. Water pressure increases with depth.
This also means that water escaping through a hole at the bottom of a dam, moves faster than through a hole at the top. The cold stream of water from a bottom discharge dam rushes into the calmer ‘pond’ below the dam. This creates a Venturi effect; similar to the air passing over an airplane wing. The lower pressure near the stream makes the water in the pond flow upstream in a semicircular path. This creates a selfperpetuating region of cold, oxygen depleted water, immediately below the dam. That is the very place where fish would spawn, and the macroinvertebrates would flourish.
Larry built a working model of a dam. He placed some coloured oil on the surface, whereupon this Venturi pattern was easy to see. The model is now with Kawartha Conservation.
9. In a natural waterway there are frequent small floods. However, the managers of the dams on the Trent Severn release large volumes of water, but less frequently. This usually occurs at the time of the spring run-off, or after periods of widespread heavy rains. This process leads to scouring, and armoring, of the riverbed. The higher energy of the sudden floods picks up, and removes, smaller sediments like silt, sand, and gravel; as well as aquatic plants and animals, leafy debris, and large woody debris.
Complex sets of habitats are erased.
The riverbed below the dam becomes like a pavement of cobbles, and loses its value as habitat for plants, macroinvertebrates, and fish.
An excellent example of this effect is seen below the Bobcaygeon dam at drawdown.
10. The natural flow of water, under normal circumstances, occurs at the surface. The bottom discharge dams create a completely alien environment in which cold, dense, anoxic water, is moving close to the bottom. Walleye require very specific temperatures at various periods in their developmental cycle. These temperatures are critical to their survival. Temperatures should not exceed 10C for egg, and sperm, maturation. Temperatures should then rise quickly up to 15C for greatest hatching success. This should be followed by summer temperatures between 16C, and 27C to maximize their growth rate. It seems likely that the bottom discharge water has a temperature lower than the 9C required for hatching success. This cold water continues to circulate in the basin below the dam due to the Venturi effect.
It is unlikely that this water will warm up in time for a successful hatch. Conversely, the warmer surface water spraying over the top of a dam on a warm spring day, could well reach that temperature. This topsy turvy aquatic environment must, on logical grounds alone, disrupt the ecosystem. In the marshes there may be impacts on turtles, and other amphibians, that have not yet been detected.
11. The Trent-Severn has another problem that could be easily resolved. The water level is lowered considerably – the draw down – as winter approaches. This makes our already shallow lakes even more so. The fish have to move into deeper ‘holes’. The oxygen levels are so low in these holes that many fish do not survive.
This is the well-known ‘winter kill’ effect. Shallow lakes with large amounts of aquatic vegetation, and mucky bottoms, are prone to this problem. They have a much lower oxygen capacity. When this is combined with decaying plants that consume the ‘bank account’ of oxygen that is left, we reach a point where some fish cannot survive.
‘Winter kill,” and water level fluctuations are dealt with in sections 5.2.3, and 5.2.4 in the Kawartha Lakes Walleye Fishery Review. These issues were clearly a matter of serious concern for the review panel. The review team discounted these drawdowns as being relatively unimportant because the walleye lay their eggs later in the season.
However, the Mayfly nymphs spend various lengths of time, up to two years, foraging on the bottom before emerging as an adult fly. It can be seen at Bobcaygeon that much of their habitat is high and dry from mid-November. This destroys a critical food source for young walleye. Mayfly is a particularly important food source for walleye fry. The calorific value of mayfly is higher than their other major food source, the amphipods. A late arriving prey population, such as the mayfly hatch, results in more predation, and cannibalism, on walleye fry. This problem is reduced when the ecosystem provides abundant forage for all fish. There has been no study to gauge the impact of drawdowns on fish populations. It would be difficult to persuade people to put up with annual flooding of their lakeshore properties, in order to carry out a controlled experiment for ten years, or so. This is probably why it has not been done so far.
12. The previous environmental impacts, taken in their totality, have caused the towns along the Trent-Severn to lose the equivalent of multi-million-dollar fish hatcheries. They have also lost the income from a thriving recreational fishing industry.
The Action Plan
Jones feels the data firmly supports the assertion that bottom discharge dam gates have had a devastating impact on a formerly healthy waterway. The taxpayers contribute millions of dollars each year to various ministries, conservation authorities, and other ecological groups.
The World Wildlife Federation comments that:
We promote sustainable dam projects in certain areas, and encourage an understanding of all the positive and negative impacts of specific dams before their construction and operation.
There is no evidence that shows that this was done for the dams on the Trent Severn according to the group.
In summary we find that the evidence suggest that two major causes can satisfactorily explain the decline of the walleye fishery:
1) The first is the annual drawdown.
2) The second is the impact of the bottom discharge dams.
Recent studies mentioned in the review show a stable situation. It is argued, on that basis, that there is no problem. However, these studies were made in the period after the sharp decline. At that point the lakes had established a new equilibrium, and no significant year over year changes are expected. But this new level of stability cannot be compared to the abundance prior to the new dams.
The reviews, and comparisons with other watersheds, looked at many possible causes of the walleye decline. But none considered the impact of bottom discharge dams. We will never return to the ideal situation with the rivers, and lakes, back in their original natural state. But we could move in that direction with some vigor, and financial support.
We all talk ecology. Here is a chance to do it.
Click here for part one: https://www.kawartha411.ca/2024/03/12/where-did-all-the-walleye-go-a-two-part-series/
Click here for Save The Walleye:https://savethewalleye.ca
