It happened near Bolin Road and Saxon Drive near US-290.
The man had gone for a walk around 11 p.m. Friday when he slipped and fell into the waterway near Benbrook Elementary School, according to Houston police.
Someone heard his calls for help and called authorities. An HISD police officer found the man, who suffered a broken hip in the fall. He was taken to a hospital around 5:30 a.m.
There was no word on the man's condition Saturday morning.