Read the river, respect the push.

The volume of water passing a point each second is the discharge, Q = A × v, where A is the cross-section area (roughly width × depth) and v the average speed. Unless water is added or taken away, Q stays the same along a stretch (continuity), so where the river gets narrower or shallower, it must flow faster. Add a steeper gradient and rocks, and you get a rapid.

Grades: how hard is this water?

Rapids are rated on the International Scale of River Difficulty, from Class (Grade) I (moving water, small waves, few obstructions) through II–III (clear channels to more powerful, irregular waves needing manoeuvring) and IV–V (powerful, long or violent rapids demanding precise control) to VI (extreme, rarely run). American Whitewater, which publishes the scale, stresses it is not exact: a river's difficulty changes with water level, and cold, high or remote water needs an extra margin. In Rishikesh, the popular Shivpuri stretch has rapids such as Roller Coaster and Golf Course (Grade III+); the Kaudiyala run adds The Wall (Grade IV), according to Red Chilli Adventure. Kolad's Kundalika runs are Grade 2–3.

Dams, rain and snowmelt

Discharge changes the river completely. Kolad's rafting depends on water released each morning from the Bhira (Tata Power) dam upstream; monsoon rain raises it further. Himalayan rivers swell with snowmelt and monsoon rain, which is one reason Uttarakhand's rafting season on the Ganga, Alaknanda, Bhagirathi and Kali runs from 1 September to 30 June under the state's rules.

Water is about 800 times denser than air. The drag force it exerts on something held still in the current is F = ½ ρ C_d A v²: water density ρ (1,000 kg/m³) times a drag coefficient C_d times the area facing the flow A, times speed squared.

What this means for you

• Don't stand up in fast water deeper than your knees. If a foot jams between rocks, forces like these can fold you over and hold you under (foot entrapment).
• Defensive swimming position: on your back, feet up at the surface and pointing downstream to fend off rocks, arms out for balance. Swim actively towards safety when the water allows.
• Boats pinned against rocks feel the same physics on a much larger area: a raft wrapped around a rock can hold tonnes of water.

Archimedes: the upward force equals the weight of fluid displaced. A human body is close to the density of fresh water, so with full lungs most of us barely float, and only part of the head stays out. A life jacket adds a few kilograms' worth of lift, high on the chest, so your face stays up. Whitewater makes this harder: aerated water full of bubbles is less dense, so the same jacket and body get less lift.

Wearing it right

• Snug, every strap fastened: a loose jacket rides up over your face. Guides check by lifting the shoulder straps.
• Helmet too: rocks are often just under the surface. Kolad's operators say every participant wears a helmet and life jacket throughout; Uttarakhand operators provide both.
• Cold water: Himalayan rivers are fed by snow and glaciers. Sudden immersion makes you gasp involuntarily (cold shock), so keep your mouth closed when you go in. Some Rishikesh operators supply wetsuits and splash jackets in winter.

Your boat moves through the water at speed b; the water itself moves downstream at v. Your path over the riverbed is the vector sum. To go straight across, aim upstream at angle θ with sin θ = v ÷ b; you then cross at √(b² − v²). If the current is as fast as you can paddle, you can't hold a line across at all.

Eddies

Behind a rock or on the inside of a bend, the main current leaves a sheltered pocket where water is slow or even circulates back upstream: an eddy. Paddlers "catch" eddies to stop, rest, scout the rapid below and wait for others. The eddy line, where fast and slow water meet, has strong shear: cross it with speed and the right edge or lean, or the current can flip a kayak.

Water falling over a drop speeds up (v = √(2 g h)), plunges to the bottom and resurfaces downstream. Part of it flows back upstream at the surface, forming a hole or hydraulic that can hold a boat or swimmer. Smooth, uniform drops such as weirs and low-head dams make the most retentive holes, with no gaps to escape through.

The hazards guides watch for

• Strainers: fallen trees and debris let water through but not people. The current pins you against them. Swim hard away; if you can't, go head-first and try to climb over.
• Undercut rocks and sieves: water flows under or between rocks where a body can be trapped.
• Holes: if caught, tuck into a ball or swim down and out with the deep current, or sideways to the edge of the hole.

A boat is stable when tilting it moves its centre of buoyancy (the centre of the displaced water) outward faster than the centre of mass moves, creating a torque that rights it. A wide raft has huge stability; a narrow kayak has little, so the paddler balances it with their hips, like riding a bicycle.

The kayak roll

Upside down, a kayaker can right the boat without getting out. A hip snap rotates the boat while the paddle gives brief support at the surface; the body stays low and the head comes up last, keeping the centre of mass near the water so very little lift is needed. Rolls are learned on flat water first: Rishikesh kayak schools such as Sea Hawk Adventures teach wet exits, eddy turns and the roll on a four-day basic course before Class II rapids.

Power on the river

Falling water carries enormous power: P = ρ g Q Δh. A 100 m³/s river dropping 2 m through a rapid releases about 1000 × 9.81 × 100 × 2 ≈ 2 MW, while a paddler can sustain perhaps a few hundred watts. You don't fight the river; you use it.