Of all the environmental changes caused by dam construction and operation, the alteration of the flow regime downstream of dams has been the most pervasive, and damaging (Worldbank, 2007). The more the flow changes, the less the ecosystem can provide the goods and services that people value it for. The impacts of such water-resource developments can stretch downstream for hundreds of kilometres, even affecting marine ecosystems (EF Window Brochure, 2003).
Bunn and Arthington (2002) highlighted four primary impacts related to flow change:
- change in channel shape and in the physical habitats such as riffles, pools, islands, and bars in rivers and floodplains, which support aquatic communities.
- disruption of biological functions by changing the volume and timing of flows.
- disruption of biological functions through the loss of longitudinal and lateral connectivity.
- the invasion of, often exotic, species that benefit from the flow changes, and thus out-compete the other species.
HPPs in particular can affect the timing and distribution of flows, and increase the rate of change between high and low flows in the downstream river. The result is mismatched flows and abnormal flow fluctuations, which can impact on both the habitat and life-cycle stages of many animals and plants (Brown and King, 2006).
A HPP will be least disruptive to a river’s flow regime when it operates as a “run-of-river” facility, with outflows essentially matching the natural regime of inflows. A run-of-river operation in its truest form would release water hour by hour at the same rate as inflows; such operations are typical of small hydropower dams with little to no storage available to modify inflows. As storage capacity increases so too does the potential effect of the HPP on the downstream environment. As a typical rule-of-thumb, a dam that creates a reservoir with a storage capacity less than 1 MAR of the river on which it is situated will have considerably less influence on the downstream flow regime than one that results a reservoir will a storage capacity greater than 1 MAR, particularly with respect to dampening of the seasonal variability, attenuation of flood peaks and delays in the onset of seasons. However, the hydrology of the river and the manner in which a reservoir is operated will also affect the extent of the influence on the downstream flow regime.
The operation of an HPP will also have a significant influence on the downstream flow regime, with peak-power generation generally having a far greater impact than baseload power generation.
Reservoirs also disrupt the natural sediment regime of rivers by trapping bedload, and coarser portions of the suspended load. Sediment supply and sediment transport capacity interact such that:
- where sediment supply is less than the sediment transport capacity, there is an excess of erosive energy, resulting in net erosion, causing the river channel to erode its bed/banks and incise; but
- where sediment supply is greater than sediment transport capacity, there is an excess of sediment, resulting in net deposition and the development of an aggrading river/floodplain environment.
Downstream of large reservoirs, water releases are largely sediment free. Sediment is replaced in the water column through erosion of the beds, banks, bars and islands, and with no opportunity for sediment replenishment from upstream, the reaches downstream of dams experience vastly enhanced erosive action relative to the pre-dam situation. In general, sediment-supply related changes downstream of dams:
- coarsening of the bed material;
- incision of the active channel/s;
- net erosion of the beds and banks of rivers due to clean water releases from dams; and
- abandonment of secondary channels and associated loss of islands, and;
- a progressive loss of habitat diversity.
Renofalt et al. (2010) provide an excellent review of the effect of HPPs on river ecosystems, and suggested mechanisms for mitigating these.