If the river bed is an even depth and snag free, try float fishing it under a small sliding pike float, shrimp pin and a small barbless treble. Aim for a delicate presentation. Hold the float back every now and then, which will lift the bait up in the water column. You could try float ledgering the shrimp, fishing it over depth using the float to move the bait around.
If the river bed is uneven, take the float off cast it out and fish it with a gently sink and draw it action. I have also caught by standing upstream of the run into a pool, cast the shrimp out, and slowly release line off the reel, inching it downstream slowly slowly.
If the water is clear and you can see the salmon, try ledgering the shrimp by casting it up stream of the fish and roll it downstream and let it settle close to the fish. Even if you don’t catch any salmon, watching them charge and circle around the shrimp is fantastic to watch.
You must strike any take immediate, as salmon can take and blow out your shrimp very quickly. Good luck👍