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Just One-Third of the World's Longest Rivers Remain Free-Flowing - EcoWatch
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Conversations with Maya: Moon Duchin. June 30, A scientist weighs in. June 28, Regardless of whether the change is good for plants and animals or not. Drained for the first time ever in So how do dams stop water flow? Dams also can only cause a certain level of water backup. Once that level is reached, water flow downstream occurs just as it did before the dam was there. If there is too little water flow downstream from a dam then there would probably have been too little water flow in the river before the dam was there.
Again, dams simply cannot totally stop water flow in a river. The dam would have to be infinitely high. It sounds like there are other issues going on besides dams. Dams change the timing of river flow, by holding back water during the spring floods and releasing that water during the rest of the year.
Normal would be high flow rates during the spring, dropping to a trickle mid summer. Post dam, the yearly flow is more constant. THis has been a big problem for the Aswan dam in Egypt, as it means the farmland downstream is no longer replenished yearly the way it was in the time of the Pharoahs. If the dam discharged from near the top, the water being discharged can be warmer than is normal. If the dam discharges from the bottom as some hydroelectric dams do , then the water is colder and lower in oxygen than it would have been absent the dam.
Cold water holds more oxygen, but only if there is a source of oxygen. Water passing through a dam usually comes out at high speed and waterfalls down a ways. The water will dissolve all the O2 it needs in that simple action. If there is little exchange of the bottom water, then the oxygen can be deleted by the biota. This happen occasionally in the lower end of Hoods Canal in Puget Sound where cold and salty water can form a rather stable puddle. The water would be stratified due to the temperature differences, and the residence time of the bottom water could be relatively large if it was not the source of the outlet flow.
The Black Sea has a similar issue. Below the level of the bottom of the Bosporus Straight the Black Sea is dead. There is no exchange of water with the upper oxygen rich layers. Some of the Fjords of Norway also have a similar issue. This however requires rather special circumstances. It is impossible in fresh water lakes in cold areas where water temperature in winter reaches 4 C.
This is the density maximum for fresh water and causes an automatic turnover of the lake water, the surface water sinks and is replaced by upwelling deep water. This is what has happened in the Black Sea, parts of the Baltic and many norwegian fjords. They are all enclosed basins with large freshwater runoff and brackish surface water. During the last interglacial which was much warmer and wetter it also happened in the eastern Mediterranean. The extra water from the Nile and other saharan and levantine rivers freshened surface water and caused anoxia in deep waters.
The lakes fill during the spring causing the dams having to release water to prevent damage to the dam. Water released from the top of the dam is no warmer than slow flowing water in the river pre-dam. Cold water released from the dam will soon warm from conduction from the earth and from the sun. There may be a some impact close to the dam. The sediment is probably not lost. As the feed river brings in sediment to the lake in the spring some of it winds up being deposited on land covered by the lake in the spring. Late planted crops such as soybeans and corn benefit greatly.
Changing the place where sediment is deposited is a change to the river. Of course they are being used for flood control. But no dam is infinitely high. Vietnam is already experiencing problems from the loss of Mekong silt. People living in Mekong Delta areas may need to buy boats as the land subsides.
Only a third of Earth’s longest rivers still run free
New Orleans and the Mississippi Delta are sinking for the same reason. When you dump silt in an area for years, the land sinks under the load. When you suddenly stop dumping silt there the land does not stop sinking. This is the result:. Steve, most river channels below dams have built in aeration. Logically, while they fill they must reduce the downstream outflow.
Once reached usual outflow will be returned. However remember that the reservoir acts as the capacitance to system to regulate outflow by absorbing excess input from upstream that otherwise would cause flooding downstream. Like wise its capacitance can be drawn down during drought. Absent this regulation on the water flow it would be incessant cycles between low water flow and flood. Reservoirs can also increase evaporation.
Both reducing the amount of water available for the river and changing the environment surrounding the reservoir. Dams do change things, good or bad another question, as I have dealt with this concerning diversions near the coast.
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You sometimes get pushed for policy actions, abdication of responsibility for those who are, well, responsible. Dams alter sedimentary processes very significantly and also affect the ability of certain types of fish to migrate. Sediment that would otherwise be carried downstream gets trapped behind dams, starving deltas of sediment. David, not all fish migrate. Exactly what migrating fish exist where the dam is located in your picture?
Where there are no migrating fish the lake resulting from the dam actually increases fish habitat and, therefore, fish populations. Nor is that sediment lost. It is merely relocated to areas above the dam.
Examples of those are in Washington State and Oregon, where they have to assist the salmon in swimming upstream. The dams on the Colorado River system, like Glen Canyon, affect sediment load. The sediment builds up behind the dam, where it often has to be dredged and it winds up starving delta systems of sediment causing them to shrink due to a lack of sediment supply.
The dams on the Lower Colorado River System have starved the Gulf of California of sediment supply and fundamentally altered the ecology of the northern Gulf of California. This sort of thing is a serious problem in the Mekong River delta. All of the flood control measures on the Mississippi River system have also reduced sediment supply to the delta system. David, the Mississippi River changes course over time as a natural change. The buildup of sediment basically creates new land mass and forces the river to find a different path to the sea.
The only thing dams on the Mississippi River would cause is to move that land formation from sediment buildup to a different location. As always, someone benefits and someone loses. While the dams on the Colorado River may have cost those in California they have benefited those along the river. Beaver dams were just an example of how natural processes affect the flow of water in a river.
I never said beaver dams were equivalent to the Boulder Dam. As a navigable river, the Mississippi is not dammed and the delta has ample sediment. This is not the case with the Mekong delta. The problem with the Mississippi is that if we stop preventing it from meandering, at some point, it will shift to the west and start building a new delta. The whole issue is that the ecology of the entire river has been changed, which you acknowledge.
The question becomes is the change overall, beneficial or not.
It is! A beaver dam in the wild creates a pond and wetlands above the dam — creating habitat for lots of species. Yes, downstream habitat will be affected but nature will evolve there also through adaptation. And sooner or later the beaver population will reach saturation and some beavers will move upstream and create a new dam and some will migrate downstream and create a new dam. In both cases, human or nature, the old delta winds up being changed and adaptation is required of all species, including humanity.
Mark, the whole issue is not whether the river ecology has been changed. The whole issue is, as you say, whether the overall change is beneficial or not. Based purely on opinion, my guess is that most water supply dams, flood control dams, and hydro-electric dams have been beneficial overall. If one understands that the inevitable destination of all land masses is the bottom of the sea, then trapping sediment behind dams could be viewed as slowing the inevitable process.
On the other hand, delta regions help slow wave erosion of coastal areas, which were previously built by silt deposition. Without silt replenishment, the delta areas quickly disappear and the wave action moves inland, to work against the rocks exposed from beneath the eroded soil. Delta regions can be protected. Doing so is a matter of economics and politics. Some smart scientists and engineers have probably already figured out how to replenish the Mississippi delta by re- introducing the sediments dredged to protect the New Orleans area, into the Delta, but then, the problem becomes political.
Those plying the ship channels speak louder than fishermen in pirogues.