Water music — Sounds and Sights

DURING my daily walks with my adopted hunting dog along the banks of a local river, under azure blue skies and in absolutely still air, I have watched peregrine falcons hover overhead and then spin vertically to catch their prey – usually a pigeon or an unsuspecting rabbit.

I have heard and seen the yellowish brown leaves float from the branches of oak trees above my head.

It is after all late autumn and early winter in the United Kingdom. As I walk the 2km-stretch of the river, every day I hear the sweet and angry sounds of this river in perpetual motion.

Suddenly my dog disturbs a pair of grey herons fishing in a pool. They fly off to circumnavigate the river valley before resting in their nest on a river-created islet.

These herons remind me of cattle egrets resting on the backs of water buffalo in Sabah and Sarawak and on wildebeests in the Masai Mara Game Reserve in Kenya.

The angry music of the river comes from the riffles on the river’s bed and the tranquil and just audible sounds further downriver in the pools. The riffling of the riffles and the relative silence of the pools.

 

What is a riffle and what is a pool?

 

Over the last 20 years, my geography students and I have surveyed the long and cross sections of this river to note the movements of pools and riffles downstream over time and in different heights of the river flow.

In 1999 and again in 2000, I tubed (sitting in an inflated car tyre inner tube) for three hours along the course of the River Danum at the Danum Valley in Sabah.

With flip-flops as paddles and an ever-swelling tiger leech on my big toe the study of the long section of this river is indelibly printed on my mind.

My KadazanDusun guide’s inner tube punctured by hitting the sharp stones on a riffle – a gravel bar.

Swirling around wildly in the pools beyond the riffles, it took a big effort to extricate my tube from deeper waters. This three-hour journey, when I was carried in the lap of a fast-flowing river, taught me more about pools and riffles than any textbook.

 

TA Stuart, a Scottish ecologist researching the spawning of salmon and trout 60 years ago, coined the terms riffles and pools. He identified the relative spacing of these natural features along the course of a river and their movements downstream over time.

Stuart astutely noted that water flowing viciously over gravel riffle bars provided aeration for the incubation of fish ova.

The American geomorphologists Leopold, Wolman and Miller, through their extensive studies, substantiated Stuart’s findings in their book ‘Fluvial Processes in Geomorphology’ (1964), namely that pools and riffles are equally spaced along the course of a river at distances between five and seven widths of a river’s channel.

Associated with gravel/pebble based riverbeds, the riffles or gravel bars are lobate (have roundish and flat arms); and in meandering channels slope alternative between banks.

The material gathered on a riffle is larger in particle size than that found in a pool. River velocity is reduced upon reaching the upstream section of a riffle only to gain a surge in energy as it rushes down the steeper downstream slope of a riffle edge into the pool below, where the river’s incoming surge of energy is dissipated as the water swirls around before moving downstream to the next riffle. Major floods cause riverbank erosion, adding further load to the river bed and ultimately to the reposition of riffles in a downstream direction.

River channels take many shapes and flowing water creates its own sounds – the riffling noise over a riffle and the peacefulness of a pool. The laminar flow of a river, with parallel layers of water shearing one over another, is a beauty to behold.

Yet when the velocity of a river exceeds a critical value, as in the time of flood, the river’s turbulent waters expend this surge of energy by eddying with still further erosion of the river’s bed and its banks.

The river is expending this energy in frictional loss against its bed and the riverbanks.

Seasonal rainfall causes excitingly different sounds in a river’s flow. Earlier this year, after a very wet autumn and winter, ‘my’ river was a raging torrent with its water spilling out over the sides of the bank onto the flood plain. It was about five times wider than its normal channel width.

Trees were uprooted and fell into the river, fence posts were knocked down as the river eroded its banks. This debris, still in the river channel, has not been cleared out by the farmers whose land the meandering river occupies.

The sound of the river at that time of flooding was nigh unbearable and the damage immeasurable in terms of the loss of farmland. So far this relatively dry autumn and winter ‘my’ river is still making sweet sounds to my ear and the riffles are moving, albeit very slowly, downstream.

See the beauty of the laminar flow of water of the Sarawak River in its downstream section in Kuching, observe and hear the force of water pouring off the waterfalls of the Lambir Hills or, indeed, for even louder water music, off the bare granite surfaces of Mount Kinabalu during heavy rainfall.

Rivers are of all shapes, sizes and volumes … each and every one provides us with different forms of beauty and its own music. Just pause and listen to water music.