CONTROL DEPTH BY ANGLING YOUR CASTS
When fishing bugs or nymphs, we can alter the depth our flies fish without changing to heavier or lighter patterns. This is done by altering the angle we present our flies. Casting squarely across flows exposes more of the fly line (where applicable) and leader to currents (diagram 9). This pressure prevents the flies from plunging too deep. Obviously, we should employ such tactics in shallow runs, or where flows are slacker. To get the very same flies to fish deeper, we should angle casts more directly upstream. Now fly line and/or leader are less affected by pushing water, allowing our flies to sink more quickly and deeper too (diagram 10).
Casting across flows exposes the whole of your fly line to flows and ushers them round more quickly to prevent them sinking too far FLOW
Casting more upstream sees far less line exposed to flows, allowing your flies to sink deeper FLOW