The river I talked about was very slow, it was controlled by sluice gates which were not opened most of the summer, although when opened when the rain was about, it did flow very strongly. However, hardly any flow most of the summer so a good weed growth developed near the edge. A few lily patches here and there as well so, a likely tench water in places. never caught that many though except for that day, and like Notskev found, often good sized tench. never caught small ones so, maybe the signs were there that they were not breeding for some reason however, the whole river has declined over the last few years, chub, bream and roach less prolific than they once were.
The type of stretches I imagine Sam Vines fish, the grayling/trout type upper stretches would not hold tench but I suspect tench exist in some of the slower deeper stretches more often than perhaps anglers know. I have heard of them in a few rivers.
Big strong fighting river tench are worth looking for or finding out about. These tench used to make my day among the run of the mill fish even if they were only a few all summer. So catching 8 on the trot that day was something else, just a fluke really but a nice fluke.
I must have caught them on bread because I only use bread and sweetcorn there because of eels and I caught these early on and I always start with bread. I have caught them on sweetcorn as well though in the river.