Multi-purpose river projects can pose which of the following environmental and geological problems? 1. Triggering earthquakes. 2. Excessive sedimentation of the river bed. 3. Making migration difficult for fishes.
The most widely accepted explanation of how dams cause earthquakes is related to the extra water pressure created in the micro-cracks and fissures in theground under and near a reservoir. When the pressure of the water in the rocks increases, it acts to lubricate faults that are already under tectonic strain but are prevented from slipping by the friction of the rock surfaces. Sichuan earthquake can be one such example. So, statement 1 is correct. All rivers contain sediments. When a river is stilled behind a dam, the sediments it contains sink to the bottom of the reservoir. As the sediments accumulate in the reservoir, so the dam gradually loses its ability to store water for the purposes for which it was built. It results in rockier stream beds and poorer habitats for the rivers' aquatic life. If sedimentation continues, it increases the water level and can even cause floods. So, statement 2 is correct. The construction of a dam on a river can block or delay upstream fish migration and thus contribute to the decline and even the extinction of species that depend on longitudinal movements along the stream continuum during certain phases of their life cycle. Mortality resulting from fish passage through hydraulic turbines or over spillways during their downstream migration can be significant. So, statement 3 is correct.