Is the area you were fishing normally 'boily' or is that something that only happens with extra water in the river?
Personally, unless I couldn't, I avoid areas of boily water when the rivers at normal level - they're normally caused by either an unseen obstruction on the bed of the river or at a point where unseen underwater currents converge - this generally causes the little whirlpools and vortices.
Despite how a river looks it doesn't all flow at the same speed and in the same direction - the flow at the surface may be considerably faster than that close to the bottom due to friction. The textbooks call it laminar flow but outside of a laboratory that's very difficult to replicate in real life - however the theory behind it is that different layers of water travel at different speeds. The problem with trying to use the theory of laminar flow in an actual river is that rivers are rarely smooth bottomed or of uniform width,depth or velocity.
When water hits an obstruction either the obstruction moves or the water moves around it, similarly when it hits the bankside - these movements cause currents beneath the surface that may not be visible or even flow in the same direction as the rest of the river and where these currents intersect you get your vortices.
You get many more of them when the river's in flood simply because the rivers carrying more water and so will generally be hitting the bankside in different places and if the overall velocity has increased some of the underwater obstruction may have been moved further downstream. Certainly on the upper reaches of the Yorkshire rivers - where you have the typical pool/riffle/pool sequence and a rocky/gravel bottom - a few winter floods can completely change whole sections.
There was a deep far-bank area of slow moving water on our old club stretch of the Swale near Catterick that was a dead cert for chub - if you could present a bait properly (long rod a necessity) - a section of bank further upstream collapsed one winter dropping a concrete slab (some sort of flood protection measure I guess) into the river and diverting the flow - within six months the deep water pool had become a gravel filled riffle less than a foot deep.
Trying to float fish in a deep river full of boils and vortices, even with a much larger float, is not easy.
Despite what the nice, fancy diagrams in the angling press show your bait will only precede your float if you hold back very hard - however hold back too hard - or use too light a float - and your bait will lift off the bottom. Also everything under the water will not be in dead straight lines - all those underwater currents that you may be unaware of will be twisting and turning your bait - perhaps lifting it off the bottom or dragging your float under - either way giving the bait a most unnatural appearance and alerting any feeding fish that there's something wrong.
Unless I could find a nice smooth, 'non-boily' bit of the river I'd fish the feeder or the lead when the rivers up - I much prefer fishing the float but sometimes it's better to present a static bait properly rather than a float fished bait badly.
Simon
