Paul
If the river comes up abnormally high, the holes of all riverbank nesting birds get swamped out and the eggs/chicks are lost.
If that loss comes early in the egg incubation period, the birds may dig a fresh hole and lay another clutch.
If however, it?s late in that period, a few days before hatching, the birds tend to give up for that year.
Where there are chicks in the nest when the river swamps the nest, it?s most unlikely that the birds will re-nest.
This is when you get a population crash, which is not normally noticed in the UK until the following year.
The nest digging strategy the bird use is to dig the holes as high up the bank as possible. There?s several advantages to this strategy, it avoids flooding out of the nest, stops predation, and gives the eggs a pretty even temperature in the hole. The latter is helpful to the parent birds, as they can feed themselves up before the onslaught of 18 -20 hour days of chick feeding. Most long distance summer visitors arrive having used up most of their body fat and energy reserves.
There is also a kind of pecking order in the colony, the more experienced/mature birds getting the heist nest holes and lest experienced first year nesters, nesting lower down the cliff face.
Whether territories are actually fought over or not, I?m not sure. My instinct is, it probably due to the experience of the birds, the old hands as it was, know where and what to do.