Isiteshi sikagesi esiphehlwa ngamanzi lapho wonke noma amanzi amaningi aphehlayo agxiliswa yizakhiwo ezigcina amanzi emfuleni.

Iziteshi zokuphehla ugesi ohlobo lwamadamu ikakhulukazi zibhekisela eziteshini zikagesi ezikhiqiza amanzi emfuleni ezakha izakhiwo ezigcina amanzi emfuleni ukuze zakhe indawo yokugcina amanzi, zigxilise amanzi emvelo ukuphakamisa izinga lamanzi, futhi zisebenzisa umehluko wekhanda ukukhiqiza ugesi. Isici esiyinhloko ukuthi idamu kanye nesikhungo sokuphehla ugesi ngamanzi kugxile engxenyeni yomfula omfushane efanayo.
Iziteshi zokuphehla amandla ngamanzi zohlobo lwamadamu ngokuvamile zihlanganisa izakhiwo ezigcina amanzi, izakhiwo ezikhipha amanzi, amapayipi okucindezela, izimboni zikagesi, izinjini zomoya, amajeneretha kanye nemishini ehambisanayo. Iziteshi eziningi eziphehla ugesi ngamanzi ezinamadamu njengezakhiwo ezigcina amanzi ziyiziteshi zokuphehla ugesi ezihamba ngamanzi ezinobukhulu obuphakathi, kanti iningi leziteshi eziphehla ugesi ngamanzi ezinamasango njengoba izakhiwo ezigcina amanzi ziyiziteshi eziphehla ugesi ngamakhanda amancane. Lapho ikhanda lamanzi lingekho phezulu futhi umfula ubanzi, indawo yokuphehla amandla ivame ukusetshenziswa njengengxenye yesakhiwo esigcina amanzi. Lolu hlobo lwesiteshi esiphehla ugesi ngamanzi luphinde lubizwe ngesiteshi sokuphehla ugesi emfuleni, okuphinde kube yisiteshi sokuphehla ugesi esifana nedamu.
Ngokuya ngokuhlobene nendawo yedamu nendawo yokuphehla ugesi ngamanzi, iziteshi zokuphehla ugesi zohlobo lwamadamu zingahlukaniswa zibe izigaba ezimbili: uhlobo lwamadamu kanye nendawo yomfula. Imboni yokuphehla amandla ngamanzi yohlobo lwedamu ihlelwe ohlangothini olungezansi lomthombo wedamu, futhi amanzi ahanjiswa ngepayipi elicindezelayo ukuze aphehle ugesi. Isitshalo ngokwaso asiwuthwali umfutho wamanzi akhuphuka nomfula. Indlu kagesi, idamu, umzila wokuchitheka kanye nezinye izakhiwo zesiteshi sokuphehla ugesi emfuleni zonke zakhiwe eduze komfula. Ziyingxenye yesakhiwo sokugcina amanzi futhi zithwala umfutho wamanzi okhuphuka nomfula. Ilungiselelo elinjalo lisiza ekongeni inani lokutshalwa kwezimali kwephrojekthi.

Idamu lesiteshi sikagesi esingemuva kwedamu livame ukuba phezulu. Okokuqala, ikhanda eliphezulu lisetshenziselwa ukwandisa umthamo ofakiwe wesiteshi samandla, esingakwazi ukuzivumelanisa ngokuphumelelayo nezidingo zokulawula ukuphakama kwesistimu yamandla; okwesibili, kukhona umthamo omkhulu wedamu lokulawula ukugeleza okuphezulu ukuze kuncishiswe ukucindezela kokulawula izikhukhula komfula ongezansi komfula; okwesithathu, izinzuzo ezibanzi zibaluleke kakhulu. Okubi ukuthi ukulahleka kwezikhukhula endaweni yedamu kuyenyuka futhi ukufuduka nokuhlala kabusha kwezakhamizi zasemadolobheni nezisemaphandleni kunzima. Ngakho-ke, iziteshi zokuphehla ugesi ezingemuva kwedamu ezinamadamu aphezulu kanye namadamu amakhulu akhiwe kakhulu ezigodini eziphakeme zezintaba, izindawo ezinamanzi amaningi angena amanzi kanye nezikhukhula ezincane.
Iningi leziteshi ezinkulu zikagesi ezingemuva kwedamu ezakhiwe emhlabeni zigxile ezweni lami. Esokuqala yi-Three Gorges Hydropower Station, esinamandla afakiwe angu-22.5 million kilowatts. Ngaphezu kwezinzuzo ezinkulu zokuphehlwa kukagesi, i-Three Gorges Hydropower Station nayo inezinzuzo eziphelele zokuqinisekisa ukulawulwa kwezikhukhula ezindaweni ezimaphakathi neziphansi zoMfula i-Yangtze, ukuthuthukisa ukuhamba kwemikhumbi nokusetshenziswa kwemithombo yamanzi, futhi ibizwa ngokuthi “imishini esindayo yezwe.”