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Rivers UnlimitedFounded in 1972 |
515 Wyoming Ave |
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Dams1. Dams slow the flow of streams as they approach the dam itself,thus reducing dissolved oxygen. Rapid flow oxygenates water. 2. Dams cause impounded waters to be permanent floods, destabilizing river banks formerly subjected only to occasional high waters for short periods, therefore causing serious and continuing erosion. The Ohio River high dam series is a painful example. Impoundments increase siltation, our number one pollutant. 3. Dams interrupt normal, seasonal flows of rivers, changing flows to suit project purposes - recreation, pollution dilution, hydropower, water supply, flood damage limitation, optimum barge traffic water levels (Ohio River). Both downstream and upstream of dams, flows can no longer serve certain species formerly adapted to natural flow. 4. Dam releases, depending on how they are controlled, can be anoxic, too cold and can cause thermal shock to organisms. These release velocities (because of the "head" or pressure of the weight of water above the downstream, tailwaters) can cause heavy erosion, generating more silt. 5. Dams collect silt. This silt therefore cannot replenish the silt constantly being removed from the streambed by downstream flow. There is no bed load just below a dam. So the streambed erodes, increasing silt. If there is no equilibrium between bedload entering a stretch of river and leaving it, either a river will cut into its streambed and deepen and/or widen it or it will fill up with silt and cut new channels through and around the silt islands. Again, instability, damage to aquatic organism habitat, erosion, silt. 6. Fish and mollusks cannot go upstream because of dams. Dams limit fisheries, even with fish ladders, which only certain fish will attempt. 7. Low flow while a dam stores up water: stream may dry up (downstream); heat up in the sun (reduces dissolved oxygen), permit algal accumulations. 8. Low flow downstream of a dam prevents dilution of fertilizer and pesticide runoff from fields and animal waste from feed lots and farms, hence increases concentration of toxics and nutrients which can damage aquatic organisms and limit their populations. 9. Dams, or rather impoundments, have limited lives. At some point they have lost significant storage capacity - they fill up with silt. They have to be dredged or drained and excavated, therefore
10. Dams may have to be removed because their condition - age or damage - may threaten downstream activities and they may cost too much to repair. It may also be in the economic interest of the community to return the river to a more natural condition to serve its original potential uses. |