generated by Kevin Okada
Effects on riparian habitats
There is a great environmental loss when a reservoir is constructed and the original fauna and flora is submerged. Vegetation and animal species are killed and there is usually no recovery plan. At the bottom of the reservoir, vegetation utilizes the dissolved oxygen until DO resources completely depleted and this region of the reservoir becomes anaerobic.
The changes in water temperature and stream flows greatly affect the fish species in California. Some of the stocks of principal California fisheries receiving efforts of preservation include the king salmon, silver salmon, steelhead trout, and rainbow trout. A method used is the control of actual instream flow (IF) and the minimum instream flow reservation (IFR). By keeping IFR conditions, the reservoir can maintain water quality for fish. The IFR is enforced by water rights permits implemented by the California State Water Resources Control Board and Federal Power Commission hydroelectric power project licenses. Fish ladders permitting the passage of migrant fish over the dam, control of downstream water flow and temperature, and artificial propagation facilities compensating for lost spawning grounds are examples of smaller size efforts to preserve the fauna.
Although efforts have been concentrated to preserve fish and wildlife, many fish still have been extinct with no replacement by other species. There is also the case where original fish stock is degraded, or reduced stock levels greatly. Other fish stocks however benefit from legislation and see an improvement on their stock. Some examples of improved fish stock are salmon and trout cited above, where efforts are being made in their preservation.
Impacts on downstream water quality
When a dam is built, temperature or heat distribution changes from free flowing conditions to a lake water environment. The thermal change has great effects on the reservoir biologically, chemically, and physically. When sun shines on the water, light can be reflected or absorbed into the water. During the fall and winter season, water temperature are cooler while during the summer season sun light heats up the surface of the water keeping cooler temperatures at lower elevations. Cooler water is denser than warmer water being densest at 4 degrees C. Some of the factors affecting thermal patterns include size of the reservoir, depth, reservoir exposure, ratio of water inflow to capacity, precipitation, and geographical location. Salinity has been a problem in many of California's reservoirs. Due to evaporation, salts can accumulate at the bottom of the reservoir increasing the water salinity.
The reservoir water is usually divided into three regions as shown in figure 1. The Epilimnion is the upper layer with warmer temperatures and turbulent flows. Thermocline is the middle region known with rapid temperature changes followed by the cold and undisturbed Hypolimnion region. The Epilimnion maintains high dissolved oxygen (DO) level due to surface aeration and photosynthetic activities. The lack of these same activities makes the Hypolimnion a region anaerobic lacking DO.
Figure 1
generated by Kevin Okada
When reservoir water is released downstream a technique used in the attempt of releasing optimal quality water combinations from the Epilimnion and Hypolimnion regions is a multi-level outlet. Optimal water quality is necessary for the fauna and flora downstream of the dam.
Morphological effects
Dams trap great deal of the suspended sediment carried by river and all of the coarse grained sediment. For this reason, the downstream river flows have a very low sediment transport rate. These sediment-starved flows naturally seek to recapture some of the lost sediment by eroding the banks and bed of the river. The riverbed degrades directly below the dam and deposits this sediment further downstream.
Also, since maximum reservoir outflows are very limited by reservoir operations, the riverbed eventually loses its capacity to move extremely large rocks. However, smaller pebbles and rocks are swept away, resulting in an "armored" riverbed that consists of many similarly sized rocks. Fish spawning rates and river-bottom invertebrate habitats can be severely impacted due to the absence of smaller coarse grained sediment in degraded riverbeds.
There are additional morphologic effects of dams that affect river channels. The degradation of the river channel directly downstream from the dams causes the river channel to become deeper and narrower. This results in the formation of straight, single channels as opposed to multi-channel braided streams. The end results are reduced channel capacity and a decrease in plant and animal diversity.