Initially, wetlands are empoldered (walled with dykes or levees) or reclaimed (drained) to make the land productive for agriculture, or to establish new human settlements, or both. Wetlands are empoldered in marine flood plains below mean sea level or in river flood plains at higher elevations. In their natural state, higher-lying wetlands serve as flood regulators. They absorb water during wet periods and release it slowly in times of drought. Empoldering destroys this function, and results in a much sharper fluctuation in river levels. Sudden spates become more frequent, the volume of river discharge increases, and the farms and townships in the lower reaches of the valleys are faced with new threats of flooding. To correct this, the river is often trained by embankments, its channel is straightened, and its bottom dredged to help the water get away. Riverside communities thus have forced upon them a stereotyped landscape with fewer natural amenities than before and an impoverishment of plant and animal life.
The same effects are felt within the empoldered land. The elimination of higher-lying wetlands results in a marked lowering of the river level in dry seasons. This can jeopardize the supply of fresh river water to those needing it. In some places, this problem is met by building reservoirs, which is a costly solution and may involve the loss of valuable land. In other places, reliance is being placed on boreholes and wells. At some of these sites, however, the groundwater resources are quickly being depleted. The replenishment of groundwater depends partly on the presence of wetlands, where the water can infiltrate into the soil. If these areas are eliminated by empoldering, the water will no longer permeate, hence groundwater reservoirs will not be recharged.
A river flowing between empoldered lands is unable to expand laterally in times of flood. As a consequence, its flow will accelerate, leading to a possible scouring and deepening of the river channel. After the flood has receded, the river, in its deepened channel, will be at a lower level than before. This can cause the empoldered lands to drain excessively into the river, or can lead to the intrusion of a salt-water tongue far upstream into the river. A river in flood carries a heavy load of silt. The low-lying lands, deprived of their natural enrichment, may then need costly applications of artificial fertilizer. In the meantime, the flood waters carry the silt to the sea, where it is thrown down as banks and bars at the estuary, encumbering the channel and comprising a hazard to navigation. Lower river levels can also affect navigation. They make it necessary to build weirs and locks to raise river levels and so maintain river traffic. Lower river discharges sometimes lead to higher concentrations of pollutants from industrial or domestic waste, high enough in many cases to constitute a threat to fisheries and public health.
The empolderment of coastal lowlands with peat or potentially acid sulphate soils is a hazardous undertaking. Peat soils are extremely unstable when not kept wet. They may shrink or dry irreversibly, offering agricultural land of poor quality. They may even disappear altogether through oxidation upon exposure to air and sunshine, incidentally producing carbon dioxide of concern for enhancing the greenhouse effect. Potentially acid sulphate soils can become extremely acidic upon exposure to oxygen, rendering agriculture virtually impossible. Such soils, under natural conditions, provide gathering grounds for natural products, and breeding grounds for fish and sea animals. The empolderment of such lands may damage these assets with little economic return.
The most serious risk inherent to impoldered lands is flooding (especially in areas with deep polders), often exacerbated by inadequate systems for warning, emergency-repair, evacuation provisions and public information. The problem occurs in Egypt, Bangladesh, Thailand, Venezuela, Japan and the Netherlands.
The occasional empolderment of a wetland need not necessarily have a severe effect on the environment. The increasing world population, however, and the growing demand for agricultural produce have led to massive empolderings that have upset hydrological and ecological equilibrium. In many cases, the ill-effects are still accumulating, since the cure to one problem is often the cause of several more.