In March and April is very cold outside so you need a flies that goes very fast in the feeding area.
A great solution for me is to use nymph tied on jig hooks.
A newbie fisherman will ask why jigs? It is simple: because they don’t snag on the bottom very easy and because these flies tied on jig hooks swim with the point up.
Models are not complicated at all but in the rivers with high fishing pressure you have to be creative. A GRHE fly can be modified a little bit using UV ribbing materials or holo tinsel.
Another jig for muddy waters:
This one is made with Pure CDC Dubbing from Trout Line because CDC have more movement in waters than standard dubbing. I use this fly in deep poles with slow running waters.
Another series of jig flies can be found here.