Read the river before you travel it: the thalweg, the line of deepest and fastest water, usually runs along the outside of bends, so go toward the outside going downstream and the inside going upstream, and keep away from strainers, fallen trees through which water flows and boats and people are trapped. Time a float over a measured distance to know the current: 1.25 m/s is 4.5 km/h, and a current of 1.5 m/s, which is 5.4 km/h, sits inside the 5-6 km/h one rower can make, so rowing upstream gains almost nothing.
Part of River Navigation & Ferry Systems
Reading a river means judging from the surface where the water is deep, fast, slow or dangerous, so that a boat can be placed where it should be.
The Surface Is A Map Of What Lies Beneath
River current is not uniform. It is fastest in the thalweg, usually on the outside of bends where the channel is deepest, and slowest on the inside, where gravel and sand pile up as point bars. Calm or reverse-flow water forms behind rocks, bridge piers and points of land (eddies); shallow fast water over gravel is a riffle; deep slow water below a riffle is a pool. The details are in Current Patterns and Depth & Bottom Reading.
Going downstream with cargo, ride the outside of the bend for depth and speed. Going upstream, stay on the inside, where the current is slower and poling is easier.
What You Need
- A float (a stick, an orange) and a measured distance along the bank, say 20 m
- A watch or a counted rhythm
- A pole or a weighted line for depth (see Depth & Bottom Reading)
- Eyes on the water: a bend, a bank and a long view ahead
Method: Size Up A Reach Before You Enter It
Step 1: Look at it from the bank first, the whole reach, bend to bend, if you can.
Step 2: Find the thalweg and the slow water on the inside of the bends.
Step 3: Spot hazards: strainers, rocks, standing waves, low dams and weirs.
Step 4: Time a float. Divide the distance by the time to get the surface speed (the average speed through the depth is lower; River & Stream Assessment applies a factor of about 0.85 to surface speed for flow estimates).
Step 5: Decide the direction and the plan, and mark where you can land if something goes wrong.
Worked Example: Can You Row Against It?
A float takes 16 s to travel 20 m. Surface speed = 20 ÷ 16 = 1.25 m/s, and 1.25 × 3.6 = 4.5 km/h. The carrier gives one rower 5-6 km/h in a 4 m boat, which is 5 ÷ 3.6 = 1.39 m/s to 6 ÷ 3.6 = 1.67 m/s. Against this 1.25 m/s current the net speed upstream is 1.39 − 1.25 = 0.14 m/s (0.5 km/h) at the carrier’s low rowing speed and 1.67 − 1.25 = 0.42 m/s (1.5 km/h) at its high one. At 1.5 m/s of current (5.4 km/h) the result runs from 5 − 5.4 = −0.4 km/h to 6 − 5.4 = +0.6 km/h: no progress to speak of. That sets the method: where the current is faster than about 1 m/s, pole or track instead (see Poling Upstream and Tracking (Cordelling)).
Common Mistakes
- Judging the river from the middle of it — look from the bank first.
- Going upstream on the outside of a bend — fastest water, steep banks.
- Following the current without a plan for the next bend — you cannot stop in a current.
Safety
Strainers and low dams kill
A strainer lets water through and holds a boat or a swimmer against it; a low dam or weir can hold a boat and a swimmer in a recirculating current. Never enter water downstream of a strainer, never run a weir or a low dam; land above it and carry the boat round. In cold water see Safety & Capsizing Recovery.
What’s Next
- Learn next: Current Patterns — eddies, riffles and pools in use
- Combine with: River Assessment: Evaluating Flowing Water — a separate check of whether the water is fit to drink