A blacktail deer drowned in the Folsom South Canal near Hazel Avenue on Tuesday morning after slipping in and being unable to climb out.
The California Department of Fish and Game tried to rescue the deer, but was unable to reach it in time.
Spokesman Patrick Foy said the deer likely became trapped Monday night but was not reported until 8 a.m. Tuesday.
By then the deer, a young buck, had become weak and injured after hours struggling to free itself from the steep concrete slopes of the canal.
"It had injuries that probably would have required euthanasia," said Foy, a game warden, who pulled the animal from the canal himself at about 9 a.m. Tuesday. Two or three deer become trapped each year in the canal, which diverts water from Lake Natoma.
This usually happens in fall, breeding season for deer, when bucks are distracted and less cautious.
The canal is fenced, but deer find their way to its edge anyway.
Foy urged witnesses to immediately report deer in the canal by calling (888) DFG-CALTIP or (888) 334-2258.