Expert: Keith Patton - 10/19/2009

Question
why is the Colorado River green?

Answer
It is probably due to algae.  Not sure what part of the river you are speaking about, but most green opaque coloration is due to algae growth.  Blue-green algae blooms occur when water temperature gets higher.  Water coming out of the base of a dam will be colder and hence you might not see the green color until it gets farther down stream and the water warms up.

Also, water carries a sediment load at higher energy states.  When it slows down or enters a placid pool, like behind a dam, the sediment load drops out.  So if you think about it, a man made dam is esentially a water fall in the making.  It is only a matter of time before the area behind the damn sediments up, and turns into a marsh, then when enough sediment has filled in, the river will flow over the sediment and down over the dam forming a water fall.  Engineers of course will try to prevent this.

When the sediment free water is released from the dam through the hydroelectric generating system, it is once again moving vigorously and guess what?  It picks up more sediment causing erosion down stream of the dam.  The high sediment load can cause the river to look muddy.  We call that turbidity and it can mask the green color of any algae until the river slows again and the sediment drops out.  But chances that the water is green coming out of the dam is slight since the water at the base of the dam or at lower levels is much colder than the surface water due to what we call a thermocline.

As the water slows and moves down river, it will warm up and the algae can once again grow.

If the water is clear and green it can be due to high levels of copper from such things as acid drainage from mines which at high levels is toxic and essentially makes the water sterile to organic live.  Copper plates and copper compound paints were used on the hulls of ships top prevent barnacles and algae from growing on them. The copper compounds absorb all the colors but green so that our eyes see the water as green, just as chlorine compounds due to the water in a swimming pool.