Skagit Double Spey
When doing a traditional sweep, that is starting low, gradual incline, rise to key position & fire, the cast is dependent upon the "white mouse" to load the rod. The "white mouse" (or white rat as I used to call it) is created by the ripping the line off the water at a faster rate than the current speed. Therefore, when in faster current the sweep must be faster.
There is another sweep not dependent on the white mouse, but on centrifugal force. The rod tip is held high all the way through the sweep, throwing the line out & around to load the rod. This method is demonstrated in depth on the Skagit Master I DVD.
On the matter of wading deep and the cast, Alexander Grant theorized that for every foot of elevation the rod tip was above the water, you gained nine feet of distance to the cast. That was a long time ago when men were men & rods were 20ft long. (and casts were 150 ft without shooting line) With that in mind, I prefer to work at depths ranging from shin to hip depth.