Any time, man. The technique I'd use would be to cast across, parallel to the bridge as close as you can get it to the bridge, then mend and mend until it has quartered downstream. Finally, use a swing technique like you were salmon fishing. Cast from near to far along the bridge until you've swung the fly past everything downstream of it. Then move downstream 30-40 feet and do it again. Rinse and repeat until fish are caught or shoulder is immobile.