I can't see a picture, so I'm assuming this fencing consists of 2 widths and one length, with the river on the fourth side.
width = w
length = w+7
So perimeter = 2 widths and 1 length
= w + w + w + 7 = 3w + 7
solve equation 3w + 7 = 61
3w = 54
w = 18 and length = 25