Ilisu Dam

This mas­sive Turkish dam, decades in the mak­ing, will be a cul­tur­al and eco­log­i­cal dis­as­ter. It fol­lows in the trag­ic foot­steps of so many mis­guid­ed dam builders and their clue­less gov­ern­ments around the world.

By 2015, the dam in­dus­try had choked more than half of the Earth’s ma­jor rivers with some 57,000 large dams. The world’s large dams have wiped out species; flood­ed huge ar­eas of wet­lands, forests and farm­lands; and dis­placed tens of mil­lions of peo­ple.

While not every dam caus­es huge prob­lems, to­geth­er the world’s large dams have left the planet’s fresh­wa­ters in far worse shape than any oth­er ma­jor ecosys­tem type, in­clud­ing trop­i­cal rain­forests. In re­sponse, dam-affected com­mu­ni­ties in many parts of the world are work­ing to re­solve the lega­cies of poor­ly planned dams. Especially in North America, com­mu­ni­ties are start­ing to take down dams that have out­lived their use­ful­ness, as part of a broad­er riv­er restora­tion move­ment.

The Perils of Excessive Dam-building

The en­vi­ron­men­tal con­se­quences of large dams are nu­mer­ous and var­ied, and in­cludes di­rect im­pacts to the bi­o­log­i­cal, chem­i­cal and phys­i­cal prop­er­ties of rivers and ri­par­i­an (or “stream-side”) en­vi­ron­ments.

The dam wall it­self blocks fish mi­gra­tions, which in some cas­es and with some species com­plete­ly sep­a­rate spawn­ing habi­tats from rear­ing habi­tats.  The dam al­so traps sed­i­ments, which are crit­i­cal for main­tain­ing phys­i­cal process­es and habi­tats down­stream of the dam (in­clude the main­te­nance of pro­duc­tive deltas, bar­ri­er is­lands, fer­tile flood­plains and coastal wet­lands).

Another sig­nif­i­cant and ob­vi­ous im­pact is the trans­for­ma­tion up­stream of the dam from a free-flowing riv­er ecosys­tem to an ar­ti­fi­cial slack-water reser­voir habi­tat. Changes in tem­per­a­ture, chem­i­cal com­po­si­tion, dis­solved oxy­gen lev­els and the phys­i­cal prop­er­ties of a reser­voir are of­ten not suit­able to the aquat­ic plants and an­i­mals that evolved with a giv­en riv­er sys­tem. Indeed, reser­voirs of­ten host non-native and in­va­sive species (e.g. snails, al­gae, preda­to­ry fish) that fur­ther un­der­mine the river’s nat­ur­al com­mu­ni­ties of plants and an­i­mals.

The al­ter­ation of a river’s flow and sed­i­ment trans­port down­stream of a dam of­ten caus­es the great­est sus­tained en­vi­ron­men­tal im­pacts. Life in and around a riv­er evolves and is con­di­tioned on the tim­ing and quan­ti­ties of riv­er flow.  Disrupted and al­tered wa­ter flows can be as se­vere as com­plete­ly de-watering riv­er reach­es and the life they con­tain.  Yet even sub­tle changes in the quan­ti­ty and tim­ing of wa­ter flows im­pact aquat­ic and ri­par­i­an life, which can un­rav­el the eco­log­i­cal web of a riv­er sys­tem.

A dam al­so holds back sed­i­ments that would nat­u­ral­ly re­plen­ish down­stream ecosys­tems. When a riv­er is de­prived of its sed­i­ment load, it seeks to re­cap­ture it by erod­ing the down­stream riv­er bed and banks (which can un­der­mine bridges and oth­er river­bank struc­tures, as well as river­side wood­lands). Riverbeds down­stream of dams are typ­i­cal­ly erod­ed by sev­er­al me­ters with­in the decade of first clos­ing a dam; the dam­age can ex­tend for tens or even hun­dreds of kilo­me­ters be­low a dam.

Riverbed deep­en­ing (or “in­cis­ing”) will al­so low­er ground­wa­ter ta­bles along a riv­er, low­er­ing the wa­ter ta­ble ac­ces­si­ble to plant roots (and to hu­man com­mu­ni­ties draw­ing wa­ter from wells) .  Altering the riverbed al­so re­duces habi­tat for fish that spawn in riv­er bot­toms, and for in­ver­te­brates.

In ag­gre­gate, dammed rivers have al­so im­pact­ed process­es in the broad­er bios­phere.  Most reser­voirs, es­pe­cial­ly those in the trop­ics, are sig­nif­i­cant con­trib­u­tors to green­house gas emis­sions (a re­cent study pegged glob­al green­house gas emis­sions from reser­voirs on par with that of the avi­a­tion in­dus­try, about 4% of human-caused GHG emis­sions).  Recent stud­ies on the Congo River have demon­strat­ed that the sed­i­ment and nu­tri­ent flow from the Congo dri­ves bi­o­log­i­cal process­es far in­to the Atlantic Ocean, in­clud­ing serv­ing as a car­bon sink for at­mos­pher­ic green­house gas­es.

Large dams have led to the ex­tinc­tion of many fish and oth­er aquat­ic species, the dis­ap­pear­ance of birds in flood­plains, huge loss­es of for­est, wet­land and farm­land, ero­sion of coastal deltas, and many oth­er un­mit­i­ga­ble im­pacts.