It begins with a distant rumbling. Within two minutes, the rumbling has turned into a thunderous roar, punctuated with loud cracks. Run is what you must do, run as far away from the stream where you are standing. And run fast! This roar is the sound of a flash flood coming towards you, and coming fast it is.
Every year, throughout the interior of Borneo, tens of people die from flash floods. People are simply caught unaware, often mesmerized by the frightening roar. Then suddenly a wall of raging water appears, rushing at you at speeds of over 80kmh. it is then too late. What makes these flash floods even more deadly is they are not always as loud as described here. Sometimes, they appear quietly, creeping up on sleeping picnickers and tourists camping by what appeared to be a seeming tranquil and peaceful riverbank.
Flash floods are destructive because of their speed and strength. They are a natural phenomenon along the upper reaches of Borneo’s rivers. Borneo’s rivers are one of this island’s most beautiful character. Over thousands of years, flowing water scours the earth, eventually reaching base rock. It is along these channels over base rock that river courses run today, forming the very bottom of valleys.
The smooth pebbles on these river beds are little pieces of art, as are the smooth surfaces of larger boulders along forest rivers. All have been sculptured over thousands of years by flowing water. They are also a clue to the power of water.
Have you ever enjoyed a picnic along a river, with shingle banks, rippling water so clear you can see fish swimming, and huge shady trees? Have you noticed accumulations of twigs, dead leaves hanging from overhanging tree branches way above you head? These are signs of previous flash floods, and seeing with your own eyes how high the water reached is a rather frightening experience.
Taking a boat ride in the interior of Borneo, passing longhouses and villages along the riverbanks, is another clue – all riverine settlements are located at least 10m above the water’s edge. Ever wondered why? Well, I guess it makes sense now.
Forested rivers are typically narrow, with rather steep sided banks rising upwards to the ridges. Huge trees overhang these rivers, creating a beautiful and shady environment. Every now and then, these trees fall down, and land in the rivers. These fallen trees influence these rivers, and can last hundreds of years. Sometimes, these tree falls land over a cascade, or small waterfall. During heavy rains, more logs and debris can accumulate at a certain point along a river, forming a dam. Behind these dams, water is then backed up.
Periodically, after periods of heavy downpours, these dams break, releasing a huge amount of water. The result is a torrent of water at the bottom of a narrow valley floor, quickly rising several metres in height, and carrying with it broken logs and trees. This is the most destructive and dangerous type of flash flood. Little in its path has a chance of surviving its tremendous force and speed. If the stream is descending a slope, this force is even greater.
Although under some circumstances these raging torrents can last several kilometres, flash floods usually dissipate as quickly as they begin. Lower downstream, river levels rise quickly, but without the roar of rushing water and breaking logs. Along narrow stretches, water levels can rise 5 metres in an hour.
Flash floods are an important part of the river ecosystem. Most of the waterfalls and cascades we enjoy so much are formed by flash floods. Following a flash flood, the river receives a big dose of nutrients from the soil and dead leaves and trees washed into the rivers. These nutrients are needed by the various lifeforms living in the rivers, from the fish, shrimp to invertebrates. In short, these floods are a necessary part of a living river.
Whether one believes in the legends of old, about the spirits of the great rainforests of Borneo showing their anger at human mischief, or hold that these destructive floods have an earthly logical explanation, the fact that they happen is undeniable. Care we must take when we head upriver. Respect the awesome power of nature at all times, and all will be well. Regardless, Borneo’s river thunder will always be a force to be respected, and many of us hope to one day be able to witness it.