How do you know the depth

Hello hope someone can help.When confronted with a swim that is too deep to plumb it is said a sliding float with a sliding stop knot is the answer,but if you domt know the depth of the water how do you know how far down the line to tie the stop knot.A typical fishery would have numerous depths all over its area,many of which you couldnt plumb Thanks.

Tie the knot carefully so you can move it to adjust the depth. I use rubber float stops, which move easily if you wet the line.

Normally only takes a couple of casts to get the depth right.

But bear in mind that Chris is a Pike-fisher, and the rubber stops that slip through his rod-rings will clatter and jam in those of a light float rod.

There are several ways to plumb up with "match"-type tackle:
1: "leading"- counting the time it takes for a lead to hit the bottom. Feeders sink more slowly than leads, giving more accurate readings; letting the weight sink freely gives a faster drop (more difficult to count) than letting it sink on a tight line, but the latter pulls it off vertical.
Crude, but shows gross variations quickly - a good first move on a new water.

2:locking sliding floats - "polaris", "loc-slide" or home-made three-ring sliders - all slide freely on a slack line, but force it round corners/curves, which lock the float in place when you tighten up.
Cast, leave to settle, tighten slowly, till the float sinks, then reel in smoothly and measure off the depth against your rod.
Inaccuracies stem from the float being pulled off vertical as you tighten, and from jerky retrieves. As with leading, each spot requires a cast.

3: The carpers' way: as large a lead as your rod will safeley cast is threaded on the line, follewd by a bead and a snap swivel; a float is clipped on to the swivel. It must be small enough for the lead to sink easily; big enough to pull line through the lead's swivel.
Cast as far as possible , let all settle without tightening, then take up the slack until the float begins to sink.
Now pull the float down to the lead, either by counting turns on the reel handle or handlining in, using a known distance on the rod as a measure (this works best with a friend to take notes).
Check your reading by feeding line back until the float surfaces. If it bobs up in the same spot, all is well; if it appears much closer in, the lead is too small, and dragging.
If all is well, sink the float back to the lead, then drag the whole shebang back by a meaured amount. Feed line till the float surfaces, check the reading by pulling the float back to the lead. Repeat until too close , then cast again.
With a big enough lead and float, this method is inch-perfect and gives several readings per cast, PLUS the chance to "feel" the texture of the bottom as you drag the lead. BUT match gear may not cope too well in anything but open, smooth-bottomed water. Best done with a beefy feeder rod - or proper carp kit.
4: The sliding float - for checking out the swim you're in.
Shot up, but omit a large shot from the bulk; put it, and another (to sink the float) by the hook. Guess a depth, tie on a stop-knot; cast and adjust as per plumbing with an ordinary set-up.
When happy, tie on a second stop-knot next to the first, and lock by coating with magic marker.While you're about it, mark the line below the float as well, so you'll see at a glance if the stops have slipped.

Test your float in the margins so it cocks just right, with a large dropper shot such as a BB. Then use the dropper shot as the plummet/tell tale. Keep sliding the stop knot up the line and by trial and error you will eventually find the depth when the float sticks out of the water more than it should. Then just allow for the length of line between the drooper shot and the hook. A little time consuming but easy when you know how.

Thanks all good stuff.

A quick way of finding the relative depth of different swims is to use a half ounce leger and count how long before it bumps on the bottom. Cast to another spot, if it takes less time to sink then its shallower.

Even more effective is to use a heavy lure such as a soft plastic shad. 1.5 ozs shads sink at about 10ft every 4 or 5 seconds. This way you get the chance of a pike as well as finding out the depth.

The count down method isn't as effective if there is a strong wind or a heavy current - the sideways pull on the line sometimes masks the moment when the weight hits the bottom.

I assume you are float fishing? If so why not use one of the locking floats like Polaris??