Let's say the river's length in the straight part of the river is x.
The total length of the river is the length of the straight part plus 3 times the length of the straight part for the crooked part, which is x+3x = 80
The total length of the river is 4x=80
The straight part of the river is x=80/4 = <<80/4=20>>20 miles.
#### 20