3. Dams

Human beings have been blocking and harnessed rivers for a variety of purposes over hundreds of years. Those purposes include irrigation, flood control and water storage. In the past hundred years or so, large dams have been built for the exclusive purpose of generation hydropower. In fact, in British Columbia, most of our electricity comes from hydroelectric dams. 

While dams can benefit society, they also cause considerable harm to rivers. Dams have flooded huge valleys displacing both people and wildlife, depleted fisheries, and degraded river ecosystems.

FOUR WAYS DAMS DAMAGE RIVERS

  1. DAMS BLOCK RIVERS AND FLOOD LARGE AREAS

    Dams flood large areas, usually valleys. This can lead to habitat loss for untold numbers of species. 

    Dams prevent fish migration. This limits their ability to access spawning habitat, seek out food resources, and escape predation. Fish passage structures can enable a percentage of fish to pass around a dam, but their effectiveness decreases depending on the species of fish and the number of dams fish have to traverse.

  2. DAMS SLOW RIVERS

    Aquatic organisms, including fish such as salmon and river herring, depend on steady flows to guide them.

    Stagnant reservoir pools disorient migrating fish and can significantly increase the duration of their migration.

    Dams can also alter the timing of flows. Some hydropower dams, for example, withhold and then release water to generate power for peak demand periods.

    These irregular releases destroy natural seasonal flow variations that trigger natural growth and reproduction cycles in many species.

  3. DAMS ALTER HABITAT

    Dams change the way rivers function. They can trap sediment, burying rock riverbeds where fish spawn.

    Gravel, logs, and other important food and habitat features can also become trapped behind dams. This negatively affects the creation and maintenance of more complex habitat downstream.

    Dams that divert water for power and other uses also remove water needed for healthy in-stream ecosystems. Peaking power operations can cause dramatic changes in reservoir water levels. This can leave stretches below dams completely de-watered.

  4. DAMS IMPACT WATER QUALITY

    Slow-moving or still reservoirs can heat up, resulting in abnormal temperature fluctuations which can affect sensitive species. This can lead to algal blooms and decreased oxygen levels.

    Other dams decrease temperatures by releasing cooled, oxygen-deprived water from the reservoir bottom.