Tulsa is built on a bend in the Arkansas River. When the river is low it looks more like the Arkansas Sandbar.
A low water dam near 21st Street keeps the river filled from bank to bank above that point. Here just below the 51St Street (I-44) bridge it dwindles according to the amount of rainfall upstream. There are plans to build two more low water dams near 51st and 71st Streets which will allow a more consistent water level in the river.