Read depth from the surface first: dark water is usually deep, light-coloured water shallow because you can see the bottom, smooth water over a bump hides a submerged rock, standing waves mark rocks or a ledge just below, and a V pointing downstream marks a deep channel between shallows. Then confirm with a sounding pole 3-4 m long marked every 30 cm, or a lead line in deeper water, because visual cues fail in murky water and glare, and compare the depth with the draft of your own loaded boat.
Part of River Navigation & Ferry Systems
Depth and bottom reading is judging from the water’s look, and checking with a pole or a lead line, how much water lies under the boat and what the bottom is like.
The Surface Suggests; The Pole Decides
The carrier’s visual cues are: dark water is deep and light-coloured water is shallow, because you are seeing the bottom; smooth water flowing over a bump is a submerged rock; standing waves mean rocks or a ledge just below the surface; a V-shaped ripple pointing upstream means a rock, with the deep channel between such Vs; a V pointing downstream means the deep channel lies between shallows. These cues work best in clear water in good light. In murky water, under glare, or at dusk, they can be wrong in either direction, so treat them as a reason to probe, never as a reason not to.
Probing also tells you about the bottom. A pole that stops with a hard click is on rock or gravel; one that sinks and sticks is in mud, which is a poor place to ground a loaded boat because it grips.
What You Need
- A sounding pole, 3-4 m long, marked in 30 cm increments (notches or bands of cord)
- A lead line: a weight (a stone in a net, a lump of lead or iron) on a marked rope, for water deeper than the pole
- A second person to watch the boat while you probe
- A note of your boat’s draft when loaded
Method: Probe Ahead
Step 1: Mark the pole. From the tip, cut a notch every 30 cm, with a different mark every metre.
Step 2: Probe ahead of the bow, from the boat, not by wading, and read the depth at the waterline.
Step 3: Use the lead line in deeper water: swing the weight ahead of the boat, let it sink, and read the mark at the surface when the line goes slack.
Step 4: Compare with the draft. Calculate it as below, and add a margin for waves and for error in reading.
Step 5: Record the depths along the route at your first passage, and again after floods, since the bed moves.
Worked Example: How Much Water Does The Boat Need?
A flat-bottomed scow 6.0 m long and 2.4 m wide carries 1,200 kg of cargo and weighs 600 kg, total 1,800 kg. Its draft is the displaced volume divided by the waterplane area: 1,800 ÷ (1,000 × 6.0 × 2.4) = 0.125 m. The 30 cm marks are coarse beside a draft of 0.125 m, so estimate to the nearest 10 cm by eye or cut finer notches for a shallow route. Loaded to 3,720 kg of cargo, the same scow draws (3,720 + 600) ÷ (1,000 × 6.0 × 2.4) = 0.30 m, and then one mark on the pole is the whole depth you need. The heavier the load, the more the depth reading matters.
Common Mistakes
- Reading the pole where the current piles water against it — hold it vertical and read at the waterline.
- Trusting a clear colour as a depth — the light colour may be a pale bottom in deep water.
- Poling by wading in current — see below.
- Not rechecking after a flood — bars move.
Safety
Do not wade moving water to check depth
A foot wedged between rocks in a current can hold a person under, and a rope tied round the waist drags a fallen person. Probe from the boat, wade only in slow water below the knee, and never tie a line to a person who is entering the river. For falls into cold water, see Safety & Capsizing Recovery.
What’s Next
- Learn next: Loading for Stability — what the draft means for the rest of the load
- Combine with: Channel Marking & Aids — recording what you find