the river's total length is and its drainage basin totals in area. the river is navigable for over . all of the water comes from the high mountains in the south where annual precipitation can be over . even before large-scale irrigation began, high summer evaporation meant that not all of this discharge reached the aral sea - though there is some evidence the large pamir glaciers provided enough melt water for the aral to overflow during the 13th and 14th centuries a.d.