Mark a navigable river channel with anchored buoys over shallows, rocks and channel edges, daymarks (posts of distinctive shape or colour) on the bank, range markers (two marks, one behind and higher than the other, that line up when the boat is in the channel) and mile stakes, and agree one convention so that everyone reads the same signs; the carrier’s rule is red on the right going upstream, “red right returning”. A boat 10 m off a range line with the front mark 200 m away and the rear mark 300 m away sees the marks 0.955° apart, 57 arc-minutes, far above the roughly 1 arc-minute the eye can resolve.
Part of River Navigation & Ferry Systems
Channel marks are the buoys, posts and alignment markers that show where the deep, safe water lies, so that boats need not read the river from scratch.
A Mark Is A Promise That The Water Is There
A mark tells a stranger that the channel is here and the hazard is there. A wrong mark is worse than none, so a mark is placed only after the channel has been sounded (see Depth & Bottom Reading) and re-checked after each flood.
The carrier lists four kinds. Buoys are anchored floats marking shallow areas, rocks or channel edges. Daymarks are posts with distinctive shapes or colours on the bank indicating the channel, hazards or distances. Range markers are two marks on the bank, one higher than the other; when they are aligned, the boat is in the channel. Mile markers are stakes or painted marks giving the distance from a reference point, essential for planning and communication.
The colour convention is a local choice and must be published. The carrier’s “red right returning” puts red on the right going upstream. International practice varies by region (red may instead be left when returning), recalled and not checked here, so anyone who has sailed elsewhere may read the marks the other way round. State the rule on a board at every landing.
What You Need
- Floats and anchors: sealed jugs or drums, rope, and a weight or anchor stone
- Posts and paint or cloth in a few distinct colours
- A surveyed centreline of the channel (see Surveying Techniques)
- Sounding gear to verify the depth under each mark
- A notice board at every landing, stating the convention
Method: Place The Marks
Step 1: Sound the channel and mark the centreline and the edges on a sketch.
Step 2: Place the range pair on the bank, aligned with the centreline: the rear mark higher and farther from the river than the front mark.
Step 3: Anchor the buoys on the shoals and hazards, with enough line to float at the highest current you expect, and tested in a strong flow.
Step 4: Put up the daymarks and mile stakes, and paint them.
Step 5: Write the convention on the notice board, and re-check everything after each flood.
Worked Example: How Sharp Is A Range Line?
A boat is at distances d₁ = 200 m from the front mark and d₂ = 300 m from the rear mark, and is a lateral offset e = 10 m from the line through them. The marks appear separated by an angle of about α = e × (d₂ − d₁) ÷ (d₁ × d₂) = 10 × 100 ÷ (200 × 300) = 0.0167 rad = 0.955°, which is 57 arc-minutes. The eye resolves about 1 arc-minute at best (a standard vision figure, recalled), so the separation is easy to see. A 1 m offset gives 5.7 arc-minutes, still visible. Moving the rear mark farther back, or the front mark nearer, makes the line more sensitive.
Mile stakes: with stakes every 1 km, a boat drifting at 4.5 km/h passes one every 60 ÷ 4.5 = 13.3 minutes, and a tracked boat at 2-3 km/h every 20-30 minutes, which lets a crew check its own speed.
Common Mistakes
- Marking a channel you have not sounded — a wrong mark is a trap.
- A buoy with too little line — a strong current drags it under.
- Different colours at different landings — one convention for the whole river.
- Never re-checking — floods move bars and break buoys.
Safety
A mark that wanders or sinks is a hazard
A buoy that breaks free drifts into the channel it marks, a tall mark on a cut bank can fall, and a taut buoy line can trap a swimmer or a boat. Anchor buoys with heavy weights and tested rope, set daymarks well back from the bank edge, and check them after every flood. In cold water, see Safety & Capsizing Recovery.
What’s Next
- Learn next: River Infrastructure — landings and docks to go with the marks
- Combine with: Signal Systems and Communication — signals to go with fixed marks