Peter,
On one occasion I stood on the banks of the Boyne River when it was in
flood, with the water level well over the banks among the gum trees.
I noticed quite a lot of splashing about and activity among the trees, and
at one point was able to go in on higher ground right to the edge of the
water.
There were swirling back eddies everywhere, and fish by the thousands, many
of them rainbows, all advancing steadily upstream.
By the roaring torrent midstream I'm sure any casual observer would have
assumed that no fish could hold on against such a flow.
I don't know for certain what happens when the flood slows, and the river
drops back to within its banks, still flowing strongly, but generally in
such conditions there are strong counter currents swirling just downstream
of bends, on the inside of the bend, so its quite possible that many of the
fish I saw could advance well upstream, and hold on to those gains.
I know there have been some thought given recently here in Queensland by UQ
to using dye marking to start some experiments on frestwater stream fish
movements.
Doug Collom