Ecological changes resulting from dam operations
At least two-thirds of freshwater flowing into the oceans is obstructed by approximately 800,000 smaller dams and about 40,000 larger dams that are more than 15m in height. In addition, many rivers are also constrained by artificial dykes and levees.
These hydrological alterations to ensure that there is water for domestic, agricultural, and industrial purposes have changed ecosystem processes and structures as far as running water, and its associated environment is concerned. This article will focus on the ecological changes in riparian ecosystems resulting from dam operations globally.
Riparian Ecosystems
Comparative studies of regulated and free-flowing rivers have increased our understanding of the effects of dams on the environment. Early works in these areas focused more on large reservoirs and aquatic systems. However, by the 1980s, rivers were viewed differently as the central components in all catchment areas. As a result, the importance of riparian ecosystems attracted more attention.
Generally, riparian ecosystems occupy the ecotone between aquatic and upland domains. A riparian ecosystem is a stream channel between the high and low watermarks plus the terrestrial landscape above the high watermark.
At the high watermark, vegetation may be influenced by the ability of soil to hold water and extreme flooding or elevated water tables. Natural riparian ecosystems include strips of spruce forest, floodplain landscapes with shrubs & deciduous trees, a forest having diverse animals and deltas containing distinct plant zonation.
One thing about riparian areas is that they are sensitive to hydrological cycles and operate as the best indicators for environmental changes caused by dam operations. Riparian processes, therefore, have a key ecological role for most landscapes. Riparian ecosystems function as filters between water and land, operate as pathways for migrating and dispersing organisms, and offer habitats for several species. The ecosystems also offer recreational and economic values.
Effects of Regulations and Dams on Riparian Zones
The environmental effects of dams are obvious and immediate. For instance, dams are known to trap waterborne sediments and obstruct migration pathways for fish.
However, other environmental effects are subtle and gradual, therefore being difficult to predict. For example, if a river is regulated, the extent of nutrient loads and floods will change, but the exact magnitude, periods and nature of these changes often remain unforeseeable.
Since every river is unique regarding the flow patterns, the species it supports, and the landscape it flows through, each dam’s operating pattern and design are also unique. This is also true for the effects of the dam on the river and the associated ecosystems. As a result, dams are normally developed to ensure less variable flows downstream.
Dams are developed to allow diversions of water which might leave channels intermittently or permanently dry. It can also reduce the flow of water very strongly. In arid areas, dams are built to ensure a continuous supply of water during the dry season. Apart from modifying the environments at the dam sites, a dam affects communities by modifying water fluctuations and raising the water level.
Upstream Effects
All the dams on the earth’s surface keep approximately 10,000 km3 of water. This is equivalent to five times the volume of water available in all the rivers globally. Storing all this water can’t be possible without far-reaching environmental consequences. This includes the formation of new riparian zones and the loss of habitat.
The upstream effect is that the volume of water increases and overpowers riparian ecosystems and terrestrial areas. This effect is very common in reservoirs close to mountains, in the far north or dry areas. Since many species in such environments are restricted to valley bottoms, a large accumulation of water is likely to eliminate the entire population. The initial effect of overwhelming the plants is through the root system. Next, the waterlogged soil will become anoxic, leading to oxygen stress and eliminating the primary root system.
The good thing is that many plants have special adaptations to cope with the oxygen stress in the soil, including the formation of adventitious roots and aerated tissue. If the plants are overwhelmed entirely, some plant species might die. Others respond by forming shoot elongation to establish contact with the open air.
Apart from just causing a loss of organisms, inundation also leads to certain environmental challenges. For instance, it might take a long time for the organic matter that was cleared to create the dam to decompose. When flooded vegetation and soils decompose, greenhouse gases such as CH4 and CO2 are released, contributing to global warming. When flooded soils decompose, they also release nutrients such as phosphorus and nitrogen that temporarily increases aquatic productivity.
Downstream Effects
Many regulated rivers contain storage reservoirs in the headwater regions. There is an intact channel that has a regulated flow downstream from these regions.
In such rivers, a single dam can affect the flow of water in almost the entire river. The changes may end up modifying riparian zones and their communities. It might also cause invasion and salinization of exotic species.
Geomorphology and Hydrology
One common effect downstream for large dams is the peaking of floods, thus reducing and displacing over time the frequency of overbank flooding.
As a result of the formation of large dams, there is increased evaporation losses and water use, leading to a reduction in downstream discharge. The groundwater table in riparian zones will also fall as a result of the altered hydrology downstream. The changes will automatically cause a reduction in the extent of the active floodplain.
There are also changes in the dam’s geomorphologic processes, including but not limited to sediment cycling. For instance, a dam developed across the Manjira river lost 60% of its storage capacity within 43 years, thanks to siltation. The water from the dam tends to restore its original nutrients and sediments, leading to increased erosion downstream of the dam. The erosion will translate into reduced geomorphologic activity and channel simplification.
Riparian Communities
Water is a scarce resource in subarctic and semi-arid regions. In these areas, floods are extremely important for fertilizing, sowing, cleaning, and watering the land. However, these areas are severely altered by impoundment.
The low altitude regions are more sensitive to water regulation than high altitude areas since the terrain is flatter. A simple alteration in the flow of water may affect large areas. Reducing or eliminating the disturbing effects of lowered groundwater level or floods that follow river regulation adjusts the composition of riparian forests to be more like those in unflooded upland areas.
The upland species are restricted from growing near and in free-flowing river channels due to intolerance to low soil fertility, physical damage, sedimentation, submersion and erosion.
On the other hand, riparian pioneer species need or are adapted to the above processes. They, therefore, have rapid germination, rapid height and root growth and easily dispersed seeds. Changes in the hydrological regime, therefore, initiates a new succession of riparian communities. Geomorphological processes, therefore, govern community dynamics.
Salinization
One major noticeable problem as far as floodplains of the Murray River is concerned is the dieback of the native tree Eucalyptus largiflorens (black box).
The primary cause of this dieback is the salinization of floodplain soils due to reduced flooding frequency in the area. The dieback occurs because naturally, saline groundwater appears on the surface when floodplain water isn’t recharged by floodwater.
Problems associated with soil salinization and waterlogging are also common in arid and semi-arid regions where reservoirs are developed to supply water for irrigation. When soils of low permeability are irrigated, groundwater tables might be forced to rise. As the water evaporates, salt will accumulate in the topsoil. The process happens even in formerly fertile regions.
Invasion by Exotic Species
There is no argument that riparian zones are vulnerable to invasion by exotic species. This is because rivers develop a natural network for dispersal across landscapes, water is available through the year, and rivers have recurrent disturbances and are dynamic. The patterns are pronounced more in regulated rivers where natural communities are disturbed further.
For instance, the regulation of river systems in South Africa has stabilized the natural hydrological regimes and led to the spread of alien aquatic vegetation, including fairy moss, water hyacinth, giant Salvinia, and parrot’s feather. The result is water loss through evaporation, reduced water movement, limited light penetration and reduced water oxygenation.
Tamarix spp (tamarisk) is a tree species that has spread significantly along regulated rivers. The species has invaded large volumes of riparian land in the United States. It has spread mainly because of irrigation and dams that have created major changes in river systems.
Future Directions and Needs
It isn’t easy to incorporate new knowledge relating to hydrological alterations on riparian ecology into policymaking. However, at the moment, there are political and economical interests that support river regulation. This has managed to minimize the ecological effects of reservoirs and dams.
There is, therefore, the need for a better understanding of the effects of hydrological alterations to determine the changes that result from already developed projects and predict possible outcomes from planned projects. The knowledge is also essential for ecological restoration, which is now becoming common. Studying the effects of hydrological alterations on riparian processes can take three directions: differences in effects between catchment areas, effects within catchment areas and changes over time.
Effects within Catchment Areas
This direction includes effects along the river and across the river. This is where the subject of the longitudinal axis comes in, which is a linkage between downstream and upstream reaches. This happens mainly in areas where the downstream and upstream effects may overlap. There is also a transverse axis that involves connections with the surrounding environments.
Effects between Catchment Areas
It’s a known fact that rivers vary in vegetation development, climate, catchment geochemistry, topography, and hydrology. As a result, they have various faunas and floras that have been exposed to several human impacts than flow regulation. The variations affect river regulation and explain why even though the same riparian processes are affected when a dam is developed, individual responses might differ considerably depending on the region. For instance, there is a variation in flooding between different regions.
Time Scales
Even though there is information on the development of riparian communities and regulated water bodies, their more stable state and future aren’t known. The reason is that most studies of succession after the closure of a dam are on rivers that have been under regulation for only a few years. Furthermore, most ecological studies last for a few years.
However, you can get some information on the long term development of riparian corridors affected by river regulation from historic evidence. You can also employ chronosequence or opt to predict environmental changes using mathematical models.
Additionally, long-term monitoring is the only reliable method for validating, assessing, and detecting predicted changes in riparian ecosystems. It, therefore, offers a helpful basis for the adaptive management of riparian systems. Finally, the most important and difficult question of our time remains how to protect the human needs of rivers and river environments.