'Flood' is an emotive word. A headache for those responsible for making sure they don't happen (but can't and know it) a potential disaster about to happen whenever the weather turns nasty for those who choose to live and work where they occur, but only ever a pain for anglers, a 'flood' is when the river rises over its banks, and it's as simple and as straightforward as that. If it doesn't burst its banks, then it's not a 'flood.'
However, a river 'in flood' is another matter. That's simply a river on the rise after rain, and no matter where it eventually reaches, even that's just a few inches up on mean levels or twenty feet over and about to start killing cows, it is a river 'in flood' on the way up in the same way that a rising tide is 'on the flood' twice a day. Oddly there's no pithy and emotive word like 'flood' for a river falling, so far as I know. There should be — as a tide falling 'is on the ebb' then a river falling should be 'in something or other' don't you think?
Both the Dorset Stour and the Itchen were 'in flood' when we arrived Saturday morning. It made fishing hard but not impossible on the Stour but perfectly feasible on the Itchen but both rivers got the same overnight rain and plenty of it. The effect upon both was remarkably different though. The Itchen did 'flood,' but it was only a very little one that you had to look hard to see and then it was just a bit of water on low lying marshy ground, but I don't think the Stour flooded at all.
Before we'd set off Saturday morning, late Friday night I checked river levels online. It had been raining for a couple of hours, The Stour had been low for some time but now was rising on the graph at the angle of an (unwanted!) erection, where the Itchen had been at solid levels for some time but was rising at the angle of a wheelchair access ramp. One looked to be going somewhere unpleasant, whereas the other looked more than manageable, and that proved to be the case.
Since then I've been looking at more graphs and they show up some interesting facts about chalk streams and mud streams and even rock streams like the Ribble, or that monster rock x mud hybrid the Severn, a floody beast that once in every 500 years wipes out its entire lowland catchment. I know I should call them by their proper names, like 'spate river' for 'rock stream,' but it's the underlying geology that makes the difference to the way they act, 'spate' being just the result of it. And besides a true spate river is a rare and very special kind where in low water conditions there's no river to speak of, just a succession of stranded pools, so the distinction needs to be made.
I digress. Here's the comparative charts for the two very different rivers we had at out disposal that weekend from Saturday morning when we arrived, till Tuesday morning as I write. Both were charted at points immediately below our respective stretches ~
|Dorset Stour at Throop - Saturday morning till Tuesday morning|
|Hampshire Itchen at Riverside Park, Swaythling - Same time frame|
The Stour you'll notice just keeps rising till it finally peaks three days later, then drops away smoothly, whereas the Itchen has already risen to its peak, table topped, and then by Saturday afternoon after Friday night's downpour, dropped sharply back to normal in just a few short hours. We saw that had already happened on arrival Sunday morning, when the levels were spot on, the brown-coloured water from the ditches had stopped coming and it was almost gin clear once more.
These rivers are just tens of miles apart, it was the same rain and in the same quantities they both got and none has fallen since, I believe, so what's happened?
The Itchen chart shows a river that has a high average level and the Stour chart shows one that has a low one. This is a condition of their respective supplies, the first fed from subterranean reservoirs in the chalk (aquifer) that keep levels healthy and relatively high till the reservoirs themselves deplete, and the second from the water table and the water that falls from the sky, saturates the ground above, and finds its way down ditches to streams to the main river.
This is complicated by run off from hard surfaces such as tarmac and roofs, which dump their water straight into pipes which feed the streams and main rivers directly. The sharp angle of the Stour's rise was due to this kind of water getting in very quickly indeed whilst the Itchen's slow rise was due to the fact that it has few hard surfaces along its length by comparison and is fed mostly by ditches when it tips it down hard.
I don't quite understand why the Stour just keeps rising for so long, though — you'd think it'd reach a peak twelve hours or so later and then fall off gradually, but it doesn't. Then again, the Stour is a longer river than the Itchen and it's a hybrid that flows through three different types of ground — an upper zone of greensand, gault and Kimmeridge clays, then a chalk zone, and finally sands and gravels with major towns like Bournemouth sited on them — so quite what that all means is anyones guess.
I suppose the Stour held all that water in the soil which then found its way into the river gradually but somehow in ever greater amounts, whereas the Itchen's water, or at least the vast majority that fell, simply vanished into porous chalk. All this is worth remembering though, should you be thinking of visiting the Southern streams — don't fish the Stour any less than three days after a torrential downpour when the Itchen will be in fine fettle just a day later.
To finish up this ramble about watery things, back to the subject of floods proper! Well here's a flood that's proper proper. I give you the Mighty Severn in full Majesty ~
In this picture the water is almost to the top of the arch, but, this is no ordinary arch — it's 50 feet wide and it's keystone is four times my height, or 24 feet above the mean level of the river, so there's near twenty feet of extra water bombing through here at a hundred thousand gallons per second. Because I've been there, admired the bridge and fished right beneath the near arch, I know how far down below that flood water I would have been, just how high above me was that keystone, and how insignificant I am by comparison with the unthinkable power of such a river as the Severn in full flood.
Astonishing, isn't it?
But it's not for fishing in...