River Regulation / Dam Construction – Effects on Rivers and Streams.
HISTORY AND BACKGROUND Large scale river regulation started 5000 years ago – to tame the flow of water and for storage purposes. Between 1900-1940 technology to build dams in N. America, Europe, S.E. Asia – E.g. the Hoover Dam – Colorado River (1936). Between 1950-1980 peak activity in dam building. From 1980-Present day, dam building has slowed to Apprx.500/yr worldwide. Estimated that at present over 60% of total streamflow is regulated.
TYPES OF RIVER REGULATION Irrigation ditches/canals and drainage tunnels Aqueducts Canals Impoundments (reservoirs) Dams
DAMS AND IMPOUNDMENTS Great variability in dams re; size, purpose and operation. These factors influence the impact on the river ecosystem. Water supply impoundments and irrigation dams need great storage capacity – to cope with demand and drought. Flood control reservoirs – small pool of water to maximise storage capacity.
EFFECTS OF DAMS (PHYSICAL & BIOLOGICAL) Physical changes downstream – flow, temperature regimes and water clarity (deep release dams most adverse effects) – therefore changes in physical and chemical conditions, resulting in changes in animal and plant life of the river. Water quality changes downstream – depends on how long water kept in reservoir or if surface/deep water released from dam. Dams break the natural upstream/downstream connectivity and periodicity of flow in rivers.
EFFECTS OF DAMS (PHYSICAL & BIOLOGICAL) Presence of reservoirs/dams affects the flow regime of the river and hence transport of fine sediments and fine particles on the streambed is altered. Inflowing sediments settle out in reservoir depleting storage capacity. High discharge dams cause scouring of fine particles and armouring of streambed (surface substrate becomes tightly compacted) as well as changes in Channel form, bank erosion, cutting down of streambed. Elimination of plants & fauna.
EFFECTS OF DAMS (PHYSICAL & BIOLOGICAL) Low discharge dams – reduced flow – increased algae/phytoplankton blooms develop and greater abundance of higher plants e.g. aquatic moss – favours midges and oligochaetes. Mayflies shift from lotic to lentic species. Reservoirs/dams impede the upstream/downstream migration of fish. E.g. salmon & eels. Damage of fish by water pressures and turbines.
EFFECTS OF DAMS (PHYSICAL & BIOLOGICAL) Decrease in species richness of benthic invertebrates but an increase in abundance – due to altered physical/chemical environment below impoundment and a reduction in habitat heterogeneity.
EFFECTS OF CANALS AND IRRIGATION DITCHES AND AQUEDUCTS Loss of channel structure and instream habitat due to altered flow – effects benthic invertebrates and fishes dependent on floods to trigger spawning. Water diversion (aqueducts) drain rivers and basins e.g. Aral Sea in Central Asia, since 1960 lake level dropped 15m, volume decreased by 60% and salinity level tripled – 24 native fish species disappeared. Canals have physical uniformity – they facilitate invasion by non-native species and loss of native species.
CONCLUSION §Dams and other forms of river regulation have many negative effects on rivers and streams, causing changes in community structure and ecosystem function. The naturally free-flowing and continuous river course is transformed into an unnatural, uniform river segment. The argument in favour of dams and many river regulation schemes is that it brings huge human and economic benefits.