Often, the aquatic ecosystem is changed because of competing uses. Sometimes to a series of negative impacts can occur because of the societal needs for water. One of the major things that could happen is hydrologic modification. This is where a stream or river is altered because of human activity. All hydrologic modifications, whether properly or improperly implemented, may result in water pollution, which will impact aquatic and riparian habitats.

Population growth and development may cause land use changes that result in hydrologic changes to watersheds. Channel modifications are sometimes needed to maintain navigable waterways and control flooding. Dam construction and operation is often necessary to store water for irrigation, recreation, and flood control and to provide a source of drinking water. Yet, these activities can be sources of pollution and adversely affect water quality and habitat if not properly managed.

One of the principal pollutants resulting from hydrologic modification is sediment. Excess nutrients and toxics may also be associated with sediment produced by hydrologic modification activities. These activities also may disturb riparian areas causing an increase in streambank erosion, and destruction of aquatic habitat.

Stream channels are often modified to straighten, relocate or change their depth and width. Once this is done, that will change:

• the instream water temperature
• the physical and chemical characteristics of the bottom sediment
• the rate of transport and sediment deposition, and
• more than likely there will be increased flooding downstream

Once the channel is changed, it needs to be maintained. This, will require dredging, which also impacts aquatic and riparian habitats for fish and wildlife. Some of these impacts may be temporary, but if there is the constant need for maintenance, then there will be long-term significant consequences.

Constructing dams and impoundments can result in significant changes in the ecology of streams. Naturally, there will be an increase of sediment and chemical contaminants. The dam operation itself will produce changes in the water temperature and chemistry (pH and dissolved oxygen). Dams and impoundments may also disrupt the natural transportation of sediment and can result in significant changes to instream flow.

The effects of flow alterations include blockage of fish migrations; alteration and loss of stream habitat; introduction of competing non-native fishes; degradation of water quality; alteration of nutrient cycles; disruption of food webs; destruction of fish eggs; decreased fish abundance, size, and condition; increased hybridization of closely related fishes competing for spawning sites; and decreased nutrient availability.

Unless streamflows are established, implemented, and protected, the following impacts can be expected to accelerate:

• Replacement of unique regional fauna by fishes adapted to more regulated stream environments. This would result in more listings of endangered species. Stream fishes currently considered as endangered would disappear in nature.
• Localized stream flooding will continue to degrade bottomlands, reduce stream productivity, and adversely affect stream fishes.
• Riparian habitat will continue to be degraded, adversely affecting stream quality.
• Reductions of streamflows will reduce and degrade stream habitat, increase summer water temperatures, reduce oxygen, and concentrate pollutants.
• Fluctuating flows associated with power generation will reduce stream resources by promoting unstable channels. Such flows will alternately scour, then promote downstream siltation of stream habitats.