Plumb the depth of a lake for fishing

It is difficult to fish effectively without knowing how deep the water is. It is also important to know the shape of the lake bed. Is it flat, does it slopes away from you or are there any sudden changes in depth. Taking ten minutes to survey and plumb the depth of the lake in front of you, will improve your chances of catching fish for the whole day.

Plumbing – what you need

To measure the water depth you will need a buoyant float and a plummet.

How to plumb the depth of a lake

Fix the float about a yard up the line with a split shot or float stop on either side. Do not add any other split shot to the line while plumbing. Pass the hook through the loop in the top of the plummet and press the hook point into the cork base.

The purpose of plumbing

With the float at the surface, the hook bait should just touch the lake bed. The distance from the float to the hook is equivalent to the water depth. The object of plumbing is to measure this depth.

Plumb the depth

Cast straight out into the lake. Give the float plenty of slack line and allow it settle above the plummet. If the float lays flat on the surface, then there is too much line between the float and plummet. If the float is pulled underwater, then there is not enough line between the float and plummet.

Reel in and move the float up or down the line, to where you think it should be. Split shot will need to be opened and squeezed back on in the new position, float stops can be safely slid along line. Cast out to the same spot and try again.

There is no need for great accuracy at this time. Later the exact depth will be found, for now just get the float in roughly the right place.

Building a picture of the lake bed

The next stage is to find out the shape of the lake bed. This is done by systematically plumbing the area in front of you. Cast out in front, let the float settle, make a mental note of the depth. Lift the rod and pull the plummet a little closer. Let the line go slack to allow the float to settle, then again make a mental note of the depth.

Once you have pulled the float close in to the bank, cast out again to a different spot and repeat the process. Cover as much water as you see fit, but build up an image of the contours, drop-off’s and shelf’s in front of you.

I like to map the lake bed by starting directly in front and working towards one side. Then going back to the front and working towards the other side.

first steps in fishing

Focus on one spot

Once you have an idea of the shape of the lake bed, you can decide exactly where to fish. Thoroughly plumb the depth in an area of about a yard around your chosen spot. Adjust the position of the float on the line, until the float shows between 1 and 2 cm of tip above the surface.

Improve accuracy

Even though we have carefully plumbed a selected area, the distance between the hook and float will be an inch or more too much. There are two reasons for this.

Firstly, it’s conical shape will cause the plummet dive point first through the water. On landing it will either fall on to it’s side or dig in to the lake bed top first. Because the plummet is not upright, it will add an extra inch of distance between the hook and the float.

Plummet lake depth

Secondly, the lake bed could be soft, muddy, or covered in leaves. It will not be firm and flat like a billiard table. The plummet will sink a little on landing, it may even sink a couple of inches if the lake bed is of soft silt.

Starting depth

A good starting point when float fishing is to have the hook bait just touching the bottom. Move your float an inch or so closer to the hook, to take account of the plummet landing on it’s side and for a soft lake bed.

Mark the depth

To make a record of the depth, secure the hook somewhere at the but end of the rod. Then using a chinagraph pencil, mark how far up the rod the top of the float comes. This is your starting depth. Having a mark showing the starting depth, saves plumbing-up again later in the day.
Amazon UK – White Chinagraph pencil
Amazon UK – Yellow Chinagraph pencil

Modify to the fish

Although the depth is now set to offer a bait just touching the lake bed, this does not mean it is the best setting for the fish. Tench and Bream for example, tip up to feed off the lake bed. It is easier for them to take a bait, if the bait and a few inches of line, are laying on the bottom. Having the bait just touching the bottom is a good starting point, but expect to adjust the setup to suit the fish.

Tench feeding

Plumbing-up is closely associated with float fishing, but knowing the lay of the land underwater, is also of great advantage when ledger fishing.

Best of luck.

Float fishing a great way to catch fish

Waggler float fishing – lake
Waggler fishing tell tale shot
Insert Waggler float fishing for shy biting fish
Driftbeater float combat wind and drift
Stick float fishing explained

View all

Float fishing articles
Fishing tips

Amazon UK – White Chinagraph pencil
Amazon UK – Yellow Chinagraph pencil