Eddies are calm or reverse-flow pockets behind rocks, piers and points of land where a boat can rest, land or wait; riffles are fast shallow stretches over gravel or rock that loaded boats can pass only at higher water; pools are deep slow stretches below riffles that suit loaded boats; and the thalweg, the fastest and deepest line, usually runs on the outside of bends. In all of them, strainers and weirs are the hazards that kill, so a river is read for those first and for speed second.
Part of River Navigation & Ferry Systems
Current patterns are the repeating shapes that flowing water makes around bends, obstacles and changes of slope, and that a boat can use or must avoid.
Each Pattern Offers Something And Costs Something
Eddies. Behind a rock, a bridge pier or a point of land, the main current separates and the water behind the obstacle is calm or flows back upstream. An eddy is where a boat rests, lands or waits for a pole. Its edge, the eddy line, is where fast and slow water meet. A boat crossing it can be spun by the difference, so enter an eddy at a shallow angle and with some speed.
Riffles. Shallow areas where the water runs fast over gravel or rock. They are often navigable only at higher water, and a loaded boat can ground on one in summer.
Pools. Deep, slow sections below riffles, safe for loaded boats and the place to rest and re-organise cargo.
Bends. The outside is deeper, faster and often undercut; the inside is shallower, slower, with gravel and sand deposits (point bars). A boat drifting on the outside is carried toward the bank at the bend’s exit, where fallen trees collect.
What You Need
- A view of the reach from the bank before entering
- Poles or oars to hold position in an eddy
- A line to hold the boat to the bank
- A plan for where each bend can be passed on the inside and each hazard avoided
Method: Use The Pattern
Step 1: Locate the eddies below each obstacle on your side; each is a place to stop.
Step 2: To enter an eddy, cross the eddy line near its top at a shallow angle to the current and with some forward speed, so the bow is carried round.
Step 3: To leave, build speed in the eddy, aim upstream a little, and cross the line at that angle; the current then turns the bow downstream.
Step 4: Take riffles on the deeper side, and look for the downstream-pointing V-shaped tongue of smooth water that marks the channel (see Depth & Bottom Reading).
Step 5: Pass bends as in Reading the River.
Worked Example: Surface Speed And Mean Speed
River & Stream Assessment estimates flow with a float and a factor of about 0.85 applied to the surface speed. If a float travels 1.25 m/s at the surface, the average speed through the depth is about 1.25 × 0.85 = 1.06 m/s. A boat with a draft of 0.3 m sits in the faster upper water, so the figure to use for a boat is the surface speed, 1.25 m/s, not the depth-average. In an eddy the same float may be nearly still or travelling upstream, so the speed contrast across the eddy line can equal the whole current speed, which is why crossing it is when boats are spun.
Common Mistakes
- Entering an eddy flat on to the line — the boat is spun.
- Following the thalweg into a strainer — the fastest line often runs under fallen trees.
- Running a weir or low dam — see below.
- Resting in an eddy that has a log in it — logs circulate with the water.
Safety
Weirs and low dams hold boats and swimmers
The water falling over a low dam turns back on itself at the foot and can hold a boat and a person against the dam; the person cannot swim out. Never run one. Land above it, carry the boat around, and keep clear of the backwash below it. Strainers (fallen trees) do the same job sideways; see Reading the River.
What’s Next
- Learn next: Depth & Bottom Reading — knowing how much water is under you
- Combine with: Poling Upstream — using the slow water on the inside