The opportunity to go swimming in a waterfall is rare anywhere. In Uganda the venturesome tourist can have the sensation of being pounded by the Nile, the world's longest river, as its waters are forced through a 20-foot-wide gap at Murchison Falls.

Thousands of gallons of water per minute lunge over the 130-foot-high waterfall. Downstream, 12-foot-long crocodiles and two-ton hippos share the water, making it impossible to swim.

Most of the river, known in this part of western Uganda as the Victoria Nile, is full of a snail that carries the intestinal parasite that causes bilharzia, the scourge of the Nilotic people because the disease results in malnutrition.

At the falls, however, the rare visitor can safely swim in a swirling pool alongside the cascading waters.

All that is required is a certain amount of caution to stay away from the edge of the chasm and the foolhardiness to be one of the few tourists in this beautiful but devastated national park, which used to attract thousands of visitors.

It also is necessary to time a visit to precede the rainy seasons, in spring and fall, since during most of the year the 20-foot diameter pool does not exist -- it is part of the waterfall.

After a decade of chaos and tribal violence, few tourists go to Uganda. Fewer still get to Murchison Falls, which despite the decimation of its elephants by poachers still has magnificent wildlife in a wondrous river setting.

If stability ever returns to Uganda and tourists flock back to the country, it probably would be impossible to dive into the circular pool that the swirling black waters have carved out of a rock formation; the pool would be fenced off from tourists.

For now, however, the courageous can swim right to the edge of the falls and watch the water cascade down through the narrow gap. So swiftly is it moving that the water is beaten into a foamy, batter-like substance that can be seen downstream for miles.

Overhead the constant spray causes rainbows, and upstream the swimmer has the sensation of watching the power of the water as it roars at him, usually veering away from the pool at the last second. Occasionally a wave will inundate the pool.

There is evidence that usually the river is much higher: a bridge over the falls has been washed away. The empty concrete abutments that once held the bridge loom above the pool like pylons for an unfinished freeway.

Hippos are the main visitors to the pool, coming on nightly sojourns during the dry season in search of fodder. Their telltale dung can be seen alongside the crumbling trail to the falls.

Although the park's facilities were heavily damaged during the 1978-79 war that liberated the country from Idi Amin's dictatorship, there are still rudimentary facilities at Paraa Lodge. The lodge was shelled during the war but has been repaired and is slowly being outfitted for tourists.

Uganda's parks "are fine for the tourist who does not like too much luxury," said warden Paul Ssali-Naluma. Paraa means "place of hippos" in the local Acholi language, and the name is appropriate. It is estimated that there are perhaps 15,000 hippos in the 100-mile stretch of river between Karuma Falls to the east and Lake Albert, which is just downstream from Kabalega.

From the rusting launch, which chugs up the Nile on one engine (mechanics were banging away on the second engine during our cruise), visitors can watch the huge hippos as they surface, spraying the river in all directions and then diving alongside the boat.

Crocodiles as long as an average room sun themselves on the bank until they hear the noise of the launch, then suddenly slither into the water in a split second. Overhead egrets, storks, kingfishers and secretary birds own the skies.

Upstream at Karuma Falls, where the cascades are almost half a mile wide, the giant Nile perch owns the river. The decaying plaque at the somewhat renovated Chobe Lodge proclaims the record catch, a 193.6-pound Nile perch on July 29, 1971.

For the avid fisherman, there is a sign to end all signs: "Perch of less than 25 pounds should be returned to the river if caught."