▁is ▁carried ▁out ▁by ▁the ▁power ▁station ' s ▁nine ▁Francis ▁p ump ▁tur bine - gener ators , ▁eight ▁belong ▁to ▁Chi ot as ▁and ▁one ▁to ▁R ov ina . ▁When ▁either ▁of ▁the ▁upper ▁res er voir s ▁needs ▁to ▁be ▁filled , ▁water ▁is ▁p ump ed ▁from ▁Pi ast ra ▁to ▁the ▁Chi ot as ▁or ▁R ov ina ▁upper ▁res er voir s ▁through ▁a ▁series ▁of ▁pen stock s ▁and ▁t unn els . ▁This ▁usually ▁occurs ▁during ▁periods ▁of ▁low ▁energy ▁demand , ▁such ▁as ▁at ▁night , ▁when ▁electric ity ▁is ▁cheap . ▁ ▁The ▁R ov ina ' s ▁upper ▁res er voir ▁is ▁at ▁an ▁elev ation ▁of ▁ ▁and ▁is ▁formed ▁by ▁an ▁emb ank ment ▁dam . ▁It ▁has ▁an ▁active ▁capacity ▁of ▁ ▁and ▁depth ▁of ▁. ▁Its ▁catch ment ▁area ▁is ▁. ▁The ▁upper ▁Chi ot as ▁res er voir ▁was ▁formed ▁with ▁the ▁construction ▁of ▁a ▁ ▁tall ▁arch - gravity ▁dam ▁which ▁lies ▁at ▁an ▁elev ation ▁of ▁. ▁The ▁dam ' s ▁thick ness ▁ranges ▁from ▁ ▁at ▁the ▁base ▁to ▁ ▁at ▁its ▁cr est . ▁Its ▁cr est ▁is ▁ ▁long ▁and ▁the ▁dam ▁has ▁a ▁struct ural ▁volume ▁of ▁. ▁The ▁Chi ot as ▁res er voir ▁is ▁also ▁supported ▁by ▁a ▁ ▁tall ▁and ▁ ▁long ▁s addle ▁dam , ▁called ▁Col le ▁Laura , ▁directly ▁to ▁its ▁east . ▁The ▁s addle ▁dam ▁is ▁also ▁equ ipped ▁with ▁a ▁sp