I have heard that their is nearly no more water going into the sea (colorado river) ?
Is that true ?
How does it goes right now ?
What should be a minimal flow to sustain the various ecological systems along the way ?
That is true, although I have heard there is a plan currently to restore a very low level of flow into the ocean. All of the water is used up for irrigation and drinking water right now (almost entirely by the U.S. before it gets to Mexico).
Dams are incredibly destructive to native ecosystems. Obviously they flood huge ecosystems under water. But equally important, the flow rate, temperature, and sediment content of the water coming out of the dams is completely unnatural. Probably the pH and other mineral content as well.
Let's compare the Green and Yampa rivers which have their confluence in NE Utah. The Green is dammed and the Yampa is not. Both rivers are of similar size. In the spring, the Yampa floods raging at 5,000-20,000 cubic feet per second (depending on the snow pack). This flushes fish eggs and minnows into downstream marshes where they mature. This year it peaked at 10,000 cfs (slightly below average). By August, the Yampa typically dries up to a mere 200-300cfs. By contrast, the Green is typically held at a constant 1,100 cfs year-round. The water coming out of the Yampa is brown with high sediment content. The sediment is important for maintaining marshes, sand banks, and underwater egg nest habitat for fish. The native fish are adapted to this murky water which gives them some competitive advantage over some non-natives. They become easy prey in clear water and out-competed by non-natives which would not be able to forage in murky water. The water coming out of the Green is clear. The water coming out of the Yampa by mid-summer is a balmy 75F. The water coming out of the Flaming Gorge Dam is 60F.
The Yampa is one of the only remaining rivers that is home to 4 endangered fish: the razorback sucker, colorado pikeminnow, humpback chub, and bonytail chub. But the limited habitat, and invasive species that have been introduced and that swim up the Yampa from the Green have put them in jeopardy. Fish researchers have not been finding ANY juvenile humpback chub in the Yampa river. The youngest humpback are usually 30 years old already. While the Yampa is a much more natural environment than the Green, unless more is done to return it to a more natural environment these fish may go extinct.
Both rivers contain tons of invasive fish (bass, brown trout, rainbow trout, and northern pike that I know of probably lots of others). The Bass and Pike are carnivorous and are a huge problem. The fish and wildlife service performs fish counts using electric shocking of the water to stun fish. They kill the bass and remove the pike from the river. I'm told bass and pike numbers are declining but still incredibly high.
There are also lots of other invasive species disrupting the ecosystem too, like the tamarisk bush which destroys natural flood plains by densely lining river banks and crowds out other native plants like the willow.
