Leon Hendry, a fourth-former at King's High School, asks :-
How do rivers run on flat ground?
Bob Spigel, a civil engineer at the University of Canterbury, responded.
It is the slope of the water surface, rather than the slope of the ground itself, that determines whether a river will flow. As long as the river water level is higher at the upstream end of a length (or "reach") of river than at the downstream end, there will be flow through the reach. Canoeists are familiar with the way deeper, slower moving water tends to alternate with swifter, shallower flow along the course of a river (a pattern called a pool-riffle sequence). The riverbed may be quite flat, or even sloping upwards, in the pools, but so long as water levels at the downstream end of a pool stay lower than at the upstream end, water will flow over the lip of the pool into the riffle downstream.
The Amazon River has its source in the Andes, and over its first 970 km it drops a vertical distance of nearly 5 km, on average more than 5 metres per kilometre. Over the remaining 5467 km to the sea, the drop in elevation is only 800 m, or about 15 cm per kilometre. This is a very flat slope, but it is enough to push the Amazon's flow (which amounts to one-fifth of the world's total river discharge) out to sea, and it is typical of the slopes of many large rivers (including the Waikato River below Hamilton). South Island rivers are steep, by world standards, even in their lower reaches. The Rakaia River drops 4.8 metres per kilometre as it flows across the Canterbury Plains after leaving its gorge below Mt. Hutt. The mighty Clutha has a milder slope, about 1 m/km from Lake Roxburgh to the sea.