Problems With Weeds in Rivers

Weeds have been described by Australia's Townsville City Council as "the wrong plant in the wrong place at the wrong time." When non-native species of plants are introduced into an ecosystem where there is no natural selection process to keep them in check, they can choke off native species of plants and animals. What might have started out as a seemingly minor alteration of a habitat, ends up compromising the natural balance of an ecosystem and setting events in motion with far-reaching ramifications. The proliferation of noxious weeds in rivers throughout the world has resulted in environmental disasters costing millions of dollars every year to combat.

Flooding

• Siltation is what happens when the roots of aquatic weeds serve to trap sediment on riverbeds. Sediment accumulates over time, reducing the depth of rivers. This diminished capacity causes the riverbanks to overflow during the rainy season and flood the lowlands adjacent to the river. The damage to farms and communities by flooding rivers is only the first phase of the problem. The seeds from noxious weeds float on the surface of the water and are carried great distances into other areas that have no natural resistance to inhibit their growth, causing the problem to grow exponentially.

Water Quality

• Excessive growth of weeds in rivers inhibits the natural free-flow of water. Water stagnation creates an environment that allows algae to multiply, robbing the water of oxygen, killing fish and other indigenous river wildlife.

Hidden Dangers

• The proliferation of invasive weeds in rivers takes place largely out of sight, under the surface of the water. The invasion of aquatic weeds in recreation areas can trap and drown swimmers as well as wildlife. The overgrowth of aquatic weeds impedes navigation by clogging boat propellers. In some cases, it can even render rivers completely unnavigable.

Control Measures

• Aquatic weeds are hardy and can thrive even when they are uprooted. Chemical, mechanical and biological methods are used to eradicate unwanted aquatic weeds depending on the type of weed to be controlled, the level of infestation and the specific environmental conditions where the infestation occurs. Chemical herbicide treatments can be costly, difficult to administer in remote areas, dangerous to other wildlife and often have only a limited or temporary effect. Biological treatment is done by introducing insects that forage exclusively on the weeds needing to be eradicated. This method has proved to have long-term success in controlling species, such as the water hyacinth, water lettuce and water fern in African countries. Mechanical harvesting is another method of control, but it is costly and provides only limited, temporary relief.

Scope

• The Food and Agriculture Organization (FAO) of the United Nations has conducted numerous initiatives and cooperative projects with experts throughout world to address the international problem of aquatic weeds in rivers and other waterways. Rivers are used extensively for navigation, fishing, irrigation and hydroelectric power generation. Rivers choked with aquatic weeds impede all these activities and, as a result, threaten entire populations in developing countries.