There are plenty of reasons to visit the Selangor river valley — jungle trekking, white-water rafting, mountain biking — but these days visitors are compelled by something more urgent.

The upper reaches of the valley will soon be deep under water, flooded by a dam that will transform the wild and beautiful area into a reservoir serving the thirsty Malaysian capital, Kuala Lumpur.

The dam project is facing opposition from environmentalists and aboriginals who live in the area, but if the government has its way, and it usually does in Malaysia, earthworks could begin before the end of the year. The dam, which the government says is necessary to secure a supply of drinking water for the capital, will take three years to complete, but once construction begins the Selangor River is expected to be awash with silt and sludge.

The underlying message for nature lovers: See the valley while you still can.

The upper reaches of the river are only 90 minutes from Kuala Lumpur's skyscrapers, but you wouldn't know it walking through the dense, pristine jungle that buffets the river as it flows down the valley, eventually emptying into the Strait of Malacca. Trails that crisscross the river and its tributaries lead to stunning picnic spots beside waterfalls and wading pools.

The proximity of such unspoiled nature to a major capital is rare for Southeast Asia, where mega-cities expanded horizontally during the boom years, chewing up jungles on the outskirts as they went.

But Malaysia, blessed with a low population density, has much to choose from when it comes to jungles. There are parks and nature preserves on the edge of Kuala Lumpur and large swaths of preserved jungle up and down the peninsula as well as in East Malaysia, across the South China Sea.

The Selangor River stands out because it is so easy to reach — from Kuala Lumpur, a simple drive up the country's main north-south highway. The area offers a range of river sports, from kayaking to white-water rafting.

There are hikes in the jungles and mountain biking on the steep, winding road that leads to what is known as the "gap," one of the few spots on the Malaysian peninsula where it is possible to cross the Titiwangsa mountain range with peaks reaching as high as 2,000 meters (6,600 feet) that separates the east and west coasts.

The road was built by the British during colonial times as a way to relay the natural riches of the east coast to the ports and major cities on the western side of the peninsula.

When the Japanese invaded colonial Malaya during World War II, they made sure to secure it in order to better control both sides of the peninsula.

Today, the two-lane road has lost its strategic significance, overtaken by bigger highways and rail lines. Mountain bikers make the grueling 30-kilometer climb up the winding road, rewarded at the top by the cooler climate of the small colonial town of Fraser's Hill.

Several companies rent equipment and run tours in the Selangor river valley, including Nomad Adventure, which runs kayaking trips, and Tracks Outdoor, which offers a range of outdoor activities including white-water rafting and mountain biking.

One of the trips run by Nomad Adventure passes the spot where the 110-meter dam will be built. Today, there are no signs of the impending project, just several small huts inhabited by the "orang asli," aboriginals who have made the Malaysian jungles their homes for thousands of years.

"It occurs to people that they want to see the river now before it disappears," says Chan Yuen Li, who founded Nomad Adventure three years ago and kayaks down the river regularly.

Chan says the river is ideal for white-water kayaking, with consistent rapids that are challenging but never too severe.

"We've explored various other rivers but none has the volume or the access," she says.

Perhaps most remarkable about the Selangor river valley are the few visitors it receives. On a recent weekend during a three-hour trek through the jungle, only one small group of hikers meandered its way to a waterfall and back, drenched by a tropical shower half-way through.

Many of the trails in the area are old logging tracks; some are smaller paths that follow streams up the mountainside. The area is somewhat of a disappointment for animal lovers, however. Unlike the dense jungles in Malaysia's national parks, frequent visitors say there have been no recent sightings of elephants, bears or tigers. But monkeys and snakes are common, as are tropical birds, which prefer the jungles higher up the slope near Fraser's Hill.

Jungle activities are not without their dangers, of course. Flash floods have swept away hikers in at least two places on the Malaysian peninsula this year.

And then there is the experience of a Dutch tour leader, Ellen van Broekhoven who, while trekking in a national park in East Malaysia, got lost and spent two nights in the jungle without any food. Soon after being rescued, she explained her ordeal.

"I believe the jungle spirits mislead me," she told a reporter for a local newspaper. "I can't find any other explanation why I got lost."

There is perhaps less risk of getting lost in the Selangor river valley; the area is a fraction of the size of Malaysia's big national parks.

And as for the flash floods, Chan, who has witnessed four and reckons they are becoming more common because of logging upstream, says visitors should be safe if they use common sense. "Pay attention to the river," she says. If it starts to rise, get out.

That, of course, is what everyone will have to do once construction of the dam begins.