Hi Matthew,
Case-1
If you want the pastures irrespective of how big they are.
1. Convert the Raster to vector (Unless you have Vectors of the same).
2. Export the pastures and river features to different layers.
3. Try Select by Location with Pastures as the target layer, and River as the source layer.
Spatial Selection Method = "Are within a Distance of"
Case-2
If you want the fraction of patches (say you want 500 meters on both sides of the river)
After step 1 and 2 of Case-1.
> Create a buffer using the river boundary for the required distance.
> Clip the Pastures layer using the above Buffer layer.
Hope it helps!!!
Think Location