1) points C and A are on the opposite side of the river from where points B and D are.
2) the angle BAC is 90 degrees.
3) the banks or edges of the river are parallel.
There are several answers:
1) if you are at point B looking at point A, then you need to know if D is 90 degrees to your left or 90 degrees to you right.
2) if you are at point A looking at point B, then you also need to know if C is 90 degrees to your left or 90 degrees to you right.
You have 4 possible solutions.
No wonder you are having trouble with it.