BOCAY RIVER, NICARAGUA -- When five Sandinista helicopters swooped down on a bluff overlooking this river May 14 with about 70 reporters from Managua, the journalists were shown an abandoned rebel post and a couple of bodies as evidence of the success of an offensive to sweep the insurgents out of this jungle wilderness in northern Nicaragua.

Now the post is back in the hands of the U.S.-backed rebels known as counterrevolutionaries or contras, who have made it a lookout position in an area they call el cuartelon, or, roughly, "the fortress."

A trip from Sept. 24 to Sept. 26 with the contras along the Honduran-Nicaraguan border and about six miles into Nicaragua suggested that, for the time being at least, the rebels again have the run of a section of the Coco River and the northern swath of the Bocay River that flows into it. Rebel posts and patrols were a common sight, and contra boat traffic motored up and down both rivers apparently unimpeded.

The trip also illustrated what continues to be a major weakness of the contras: their almost total dependence on U.S. aid. At a position on the Honduran bank of the Coco River, which forms the border with Nicaragua, a supply plane made two airdrops of food an hour apart on Sept. 24.

These supplies of flour, rice, beans, powdered milk and other items are distributed to other positions along the Coco and Bocay rivers, providing the major source of food for contra combatants and civilians living in jungle locations inaccessible by road.

Contra leaders expressed concern mixed with disbelief that these and other vital supplies might soon be cut as part of a Central American peace plan.

"If the aid is cut, our alternatives are very few," visiting contra political leader Adolfo Calero acknowledged during an interview Sept. 25 on the Nicaraguan side of the Coco River. "We can stretch out our munitions, but the food we cannot stretch out."

Although the contras appear to have put some effort into returning to their posts in the northern Bocay region, the importance of this wilderness is open to question. The contras described the Bocay as a strategic resupply zone when they held it in April. But when the Nicaraguan Army invaded it in May with an estimated 3,000 helicopter-borne troops and expelled the rebels, contra leaders said the Sandinistas had been lured into a useless, inhospitable area that they would find costly to hold.

The Bocay does seem to pose a dilemma for the Sandinistas. On the one hand, the propaganda value of the contra presence here appears to irritate the Managua government. But the area is difficult for the Sandinistas to occupy and keep resupplied, given the absence of civilian inhabitants and food sources following massive Sandinista relocation campaigns a few years ago. Now, helicopters bringing in food and other supplies risk being shot down by the contras' U.S.-supplied Redeye shoulder-fired ground-to-air missiles.

The Redeyes are among several pieces of new equipment seen since a visit in mid-April. The contras also have some new American flat-bottomed plastic fishing boats fitted with outboard motors that they use to zip up and down the Coco River. In addition, there is new mine-sweeping equipment to detect the hundreds of Soviet-supplied land mines left behind by the Sandinistas when they left the area in August.

At the contra post shown to journalists by the Sandinistas in May, empty cans of Soviet rations and Hungarian goulash testify to the former Sandinista presence. There are also empty wrappers of U.S. iodine pads, marked PVP-I Prep Solution, that a contra official said had been given to the Managua government as humanitarian aid by pro-Sandinista organizations in the United States.

Also left behind were pages of Sandinista literature used for the political indoctrination of Sandinista Popular Army troops. One tract on political parties condemns the U.S. two-party system and criticizes "bourgeois" opposition parties in Nicaragua. In Nicaragua's "political pluralism," it says, there is room for "political organizations of the bourgeoisie, although political hegemony belongs without discussion to the working masses."

According to this zone's contra commander, who goes by the code name Dumas, the Sandinistas now are located about 18 miles up the Bocay River from their former position here, which is about six miles from the Honduran border. He said the Sandinistas had to abandon the captured post around Aug. 10 because of harassment from his men and resupply difficulties.

As he spoke, the sound of a mine explosion reverberated in the distance. But it could not be ascertained whether someone had stepped on the mine, or whether it had been deliberately detonated by the contras.

At a former contra camp on the Honduran bank of the Coco River opposite the mouth of the Bocay, rebel mine-sweeping units worked to clear a landing strip that was heavily mined by the Sandinistas after they captured it in the May offensive. The strip had been used to drop supplies by parachute and to land small short-takeoff-and-landing planes ferrying contra officials and journalists to the zone.

Now journalists arrived at a nearby contra camp by helicopter after a hair-raising flight that wound through uninhabited jungle valleys at treetop level to avoid detection by the Sandinistas and exposure to missiles from the Nicaraguan side of the border.

Although the strategic importance of the Bocay is debatable, it does seem to serve at least one useful purpose in speeding the evacuation of wounded contras.

Among those plying the river in motorized dugout canoes Sept. 25 was a 47-year-old contra codenamed Minero. Wearing crude bandages around his chest and speaking with difficulty, the former gold miner from the town of Siuna said he had been hit by shrapnel from a Sandinista mortar in a battle in northern Zelaya province Sept. 13. It had taken him 12 days to get to the Bocay, and now he was on his way to a contra field clinic with seven comrades and two boatmen.

According to Braulio, a doctor at the jungle clinic, contra officials are preparing for the possibility of a cutoff of U.S. aid after Nov. 7, when a Central American peace plan is scheduled to take effect. A $100 million U.S. aid package with an expiration date of Sept. 30 has been supplemented by $3.5 million to bridge the gap until Nov. 7. But after that, if the peace plan's provisions for cease-fires, amnesties and democratic reforms are adhered to, the contras' aid is to be cut off and their military bases removed from Honduran soil.

"We are ready for an aid cutoff," Braulio said. "We are trying to make our medicine last longer." He added, however, that he did not believe the Sandinistas would comply with the democratization provisions called for in the peace accord, because "it means they would lose power."

Pitufo, a paramedic whose code name means Smurf in Spanish, said, "If the aid is cut off, it's going to hurt us a lot . . . . We would have to look for other sources." He paused, then added, "But I can't believe Ronald Reagan would leave us without aid."

Pitufo said he joined the contras in 1983.

"I wanted to do something for my country," he said. "We in the resistance love democracy. So we are obliged to abandon everything and take up arms."

For contra commanders like Mercenario, the operations chief of this zone, an aid cutoff would mean more hardship but not an end to the struggle. A former intelligence officer in the Nicaraguan National Guard, Mercenario said he can never return to a Sandinista-controlled Nicaragua and must fight on regardless of outside support.