As stated there are some assumptions:

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.