15.4 EFFECTS ON ECOLOGY

Removal of obstructions to navigation and improvement/construction of port facilities can have significant impacts on the river ecosystem.

Removal of rocks and rapids, and channel dredging, can cause permanent alteration of fish habitat, as well as reduce ecologically important habitat complexity and diversity. Some rocky areas, as well as rapids and shallows are critical for life cycle activities of some species, for example providing important areas for spawning and rearing (see Section 13).

As an example of ecological importance of some areas that are being considered for modification, fish species, such as the giant catfish, migrate long distances from areas in Cambodia to spawn in the complex habitat formed by deep-pools and rapids found in portions of the river along northern Thailand and Lao PDR. Removal of rocks to improve navigation in this area may reduce numbers of giant catfish and livelihood of people that catch them in distant downstream areas. Also, blasting for rock removal has potential to harm large numbers of fish, mammals and other wildlife using nearby water, so timing of such activity must be carefully chosen to minimize damage.

Channel dredging can alter water quality, most noticeably by greatly increasing the quantity of suspended solids, but also by increasing the amounts of dissolved substances. Dredging sometimes results in the release of contaminants that have accumulated in bottom sediments. Changes in the concentrations of solid and dissolved material can in turn affect plant and animal life downstream. Increased sedimentation in shallow areas can affect navigation in addition to disturbing aquatic organisms.

Placement of dredged material is an important concern, especially if the material is placed in areas of the river where it may eliminate deep pool habitat that is important for some species. Care must be taken if material is placed on land so that material does not chronically re-enter the river during the rainy season.

Any change in upstream hydrology can cause changes in downstream conditions, sometimes leading to unintended alteration of flood patterns and riverbank erosion. Changes in river morphology that results from obstruction removal can change flow characteristics, making habitat less useable for key species at important areas downstream. Lock construction and operation can change flow patterns and impair fish movement. Placement of water control structures such as dams and reservoirs, designed to maintain increased water depth over the dry season, will alter seasonal habitat features.

During construction, additional changes in water quality may result from equipment and construction activity, for example oil and lubricant discharge from machinery and domestic waste from construction camps.

Improvements to navigation or port facilities will likely increase the number or size of vessels moving along portions of the river. This can lead to increased noise, contaminants such as oil discharge, and wake effects on the shoreline and shallow bottom areas.