River runners, like mountain climbers, belong to an exclusive fraternity. The higher the mountain, the greater the challenge. The rougher and more remote the river, the greater the dare.

Sobek Expeditions has met this challenge with an exciting and sometimes hazardous three-week-long, 350-mile journey by oar-propelled raft on the Omo River in southwest Ethiopia. The goal is to visit the country of the Bodis, a primitive african tribe that has lived in isolation for unknown generations.

When my wife and I joined this expedition, we were forewarned to be prepared for discomfort and the hazards of hippo and crocodile attacks on the rafts. We were not misled.

Before the trip, Mountain Travel of Albany, Calif., very wisely sent along a list of "don'ts" for boating on the Omo -- don't camp on hippo trails; don't go near the water after dark (that's when most crocs feed); don't provoke the hippos; don't hike or swim alone, and don't drink the water until it has been treated.

We took a jet to Athens, Greece, where we made connections with Ethiopian Airlines, which flies directly south to the capital of Ethiopia, Addis Ababa.

With a war against guerrillas on two fronts, a military junta in power, high unemployment and serious gas and food shortages, Addis seethed -- but below the surface. The army was out in force, and huge portraits of Marx, Lenin and Engels adorned walls. Other posters described Uncle Sam being mercilessly beaten by enraged citizenry. The Ethiopians on the street, although not openly hostile to their regime, watched indifferently as new Russian equipment rumbled through the streets on its way to the Ogaden front.

The expedition was compltely self-contained. We brought all our food with us on the boats -- enough to last, we hoped, for three weeks. Tents and clothing were stored in waterproof bags. There was no way out, no turning back from the Omo until the Mui game reserve, far to the south, was reached.

Our expedition was made up of five rafts and six oarsmen. Sobek Expeditions uses 18-foot Avon rubber rafts that can carry four persons and supplies in reasonable comfort. They also supplied oarsmen -- all young, experienced American river runners, usually a few with previous experience on the Omo.

No one else seems to travel on the Omo, so we had it to ourselves -- if you don't count its rather sizable population of hippos and cros, which often present a problem. Once a crocodile did attack a raft and was driven off only after being beaten over the head with an oar and receiving a barrage of rocks. All boats carried "croc rocks" for this purpose.

The hippos are just as formidable, but easier to see and avoid. On previous trips they had been known to attack and puncture rafts. But for us they only announced their annoyance at our presence with loud snorts, and glared balefully at our boats gliding by. On one day alone we dodged 375 of these monsters; by the end of the trip we had sighted more than 1,200 of them.

The Omo River is truly unspoiled beauty. It tumbles and spumes its way over scores of cataracts, cutting deep gorges through the Ethiopian highlands to spill out into the savannahs of the Omo Valley. Finally it empties into Lake Rudolph.

At night we pitched tents along the tropical banks. The air was alive with the sounds of baboons barking, the cries of fish eagles, the hippos snorting in the river and the hurried murmur of the Omo pushing her way south.

Daytimes we often suffered climatic extremes, ranging from intense tropical heat to chilling rainstorms, but on the whole the weather was surprisingly comfortable, usually in the middle 80s. Insects kept our party in shirts and pants most of the time, however.

The river was the color of chocolate and running high and swift, swollen from the nightly thunderstorms. The many rapids were exciting tests of the oarsmens' skills, and rivaled anything the Grand Canyon gorge has to offer.

We spotted our first Bodi on the 15th day. A tall, spear-carrying warrior hailed us from the river bank, shouting "Salam, salam." The expedition had reached the land of the Bodis, whom some anthropologists consider to be among the most primitive and isolated people in Africa today.

Unusally shunning clothing, the Bodis adorn themselves with decorative scarring, ear and nose rings. A few women still wear the clap lip plates, although this is a dying practice and those we saw with discs hid their faces in embarrassment.

Actually, there are three tribes inhabiting the Omo Valley: the Bodis, the Mersis and the Bachas. All together, they probably number no more than 3,000 people.

The Mersi is the largest of the tribes. Mersis are more sophisticated, given to occasional wearing of scanty clothing and more successful in their "cattle wars" with the other tribes. Some of their men were armed with ancient Mauser rifles. Ammunition, was so scarce, however, that only few bullets were in evidence.

The Bodis are hunter-gatherers. Usually armed with long spears, they are likeable and friendly and, when given a chance, great orators. My wife introduced them to a tape recorder and once they heard their own voices, nothing could turn them off.

The Bachas are believed to be a fairly new tribe in the Omo Valley -- perhaps (although nobody knows for sure) coming from the nearby Sudan. They number no more than several hundred, and appear to be poor and exploited by the other tribes. Once a Mersi tribesman referred to the Bachas as "those monkey people."

Although the Bodis rarely see foreigners they have become accustomed to the once-a-year Sokek expeditions, and they have learned that the name of the game is trade. We traded our soap and razor blades for their native artifacts. We also posted guards at our campsites to keep them from pilfering us blind, but it all was a game, and if we caught any of them we treated it as a huge joke.

Once I discovered a young man trying to steal my ground pad, which he had wrapped around him. We both enjoyed a laugh while I retrieved it, but my laugh had a hollow ring, since he was armed with an eight-foot spear and my heaviest weapon was a fly swatter.

A week was spent visiting with these tribespeople, who still start their fires by the twirling-stick method and grind their sorghum in stone pestles.

Sobek Expeditions arranged for a DC-3 charter to pick up boats, passengers and crew at the Mui game reserve at journey's end, but warned the expedition members to be flexible about their schedules -- for good reason. Our group waited three days for our aircraft. Bad weather had held it up. At this point our rations had been reduced to a bag of granola and a goat we had purchased from one of the game guards at Muri.

Memories of the animals, the exotic birds and the colorful tribes that make the Omo unique will remain with me forever. Here is the heart of Africa, the old Africa -- unspoiled, timeless, remote and still splendidly primordial.

(Sobek has been wholesaling adventure travel since 1968, and has run trips to Ethiopia every year since 1973. Last year it became a retail travel agency and thus is now permitted to book its own packages, but they can also be booked through other agents.

(A spokesman for Sobek said the political situation in Ethiopia will not prevent them from operating two river-running trips on the Omo this October and November. He added that Ethiopian Airlines reports the country is opening additional areas to tourism. Sobek's address is Box 67, Angels Camp, Calif. 95222.)