Flow and Flood Reduction:

The reservoir a mega dam creates not only has dire consequences on the environment upstream, but also negatively affects the lands downstream.  A mega dam has the ability to carefully regulate water flow downstream, obscuring its natural run.  Watercourses can be so constrained by the dam that water can be nearly non-existent at the mouth of the river, reducing the amount of water available to plant and animal species living downstream and hindering their ability to thrive.

Additionally, before the 1970s dams were constructed so that water was released from the bottom of the reservoir.  This water, which is cold, depleted in oxygen, and low in sediment concentration, is much different from the water that would have naturally flowed downstream.  Consequently, this different water composition has adverse effects on species downstream, particularly vegetation.  Environmentalists brought enough attention to this problem that now old dams are being modified so that warmer more oxygenated water is released from the top the reservoir (Woodward).

Reservoirs also retain water preventing annual flooding during rainy seasons, which is often detrimental to animal and plant species that depend on the benefits of short term but intense flooding.  Though, this ability to control floodwaters does offer several humanitarian benefits and may have been the initial purpose of dams, it also negativity impacts traditional farming practices.  Farmers in Egypt and Bangladesh for example, depended on seasonal flooding to carry nutrient rich sediments to fertilize their lands.  Additionally, the decrease in water pressure prevents surface water from seeping into natural watertables downstream from the dam, causing natural water sources to become barren, hindering agricultural production (WWF).

Annual floods also have the ability to wash away obstructions along the course of the river, which prevents larger floods from creating catastrophic damage.  Without these annual floods, material is allowed to buildup as a result of the dam.