The problem is becoming greater with increased trade and development which tend to spread and encourage the growth of weeds. In particular, man-made lakes, reservoirs, canals and irrigation systems, with their different ecology from natural water courses, often provide an ideal environment for the spread of aquatic weed, especially where the water is shallow and rich in nutrients. The concentration of human settlements, the development of sewage systems and the increasing use of fertilizers ensure that large amounts of nutrients are entering water courses, there to stimulate weed growth.
Aquatic weeds are spread very quickly through the medium of the river. Boats distribute weeds up- and downstream. Currents carry them downstream and floods push them into pools and swamps and backwaters. Seeds may be carried by wildlife and the wind. Man-made water systems are particularly susceptible to weed growth, as man-made reservoirs and distributaries are often shallow and clear, as well as rich in nutrients, and weed growth follows. For example, submerged aquatic weeds have cut the flow in one large irrigation canal system in India by 80%. The result is that the reduced flow encourages seepage from the canal and thus contributes to water-logging and salinity. Finally, water cannot be moved to crops, animals, and people in sufficient quantity, on schedule.