Aground on the Mississippi. On the lower half of the Mississippi, drought has hit with full force. Some barges have run aground and the river industry is hurting. Streamflows in rivers in more than half the country are well below normal. Thunderstorms rained on the upper Midwest yesterday, but long-term relief looks elusive.

THE Mark Eastin, pushing 15 barges of corn and wheat down the Mississippi River, slips quietly into Lock No. 26. ``Where are you headed?''

``Blue Island,'' a deckhand answers.

``Will you get there with all the low water?''

``One of these days,'' he says.

This section of the Mississippi represents a turning point for the tow boats and barges that ply these waters. Upriver, a series of dams and reservoirs have kept water levels adequate for navigation, despite the severe shortage of rain. Downriver, just a few hundred feet, a dredge is working to keep the channel open.

On the lower half of the Mississippi, the Midwest drought has hit with full force, causing barges to run aground and, in a sense, the river industry itself.

``It looked like it was going to be a pretty good year,'' says Bob Elliott, a trip pilot for the Mississippi River.

After struggling with too few grain exports during the first half of the 1980s, the barge industry was expecting the United States' expanding export sales to bolster a recovery. But long delays, caused by grounded barges and low water levels, are cutting into this year's profit margins, Mr. Elliott says. And the drought may reduce the crops that the industry would normally carry this fall and next spring.

On Wednesday, for example, a tow with barges ran aground some 80 miles downriver from here, near St. Genevieve, Mo., closing the river. The river is also closed six miles upriver from Memphis. Since Tuesday, the US Army Corps of Engineers has been dredging there to clear a passage. At press time, the dredging was scheduled to be completed by this afternoon.

``We've probably seen several months worth of marine incidents occur in the last two weeks,'' says Petty Officer Jim Pogue, spokesman for the US Coast Guard group based in Memphis. The group, responsible for much of the lower Mississippi, the Arkansas River, and several Mississippi tributaries, has seen some 750 tows grounded since the drought situation began.

``It's unusual for anytime in the season,'' Petty Officer Bogue says. Two weeks ago, the river was closed for several days near Greenville, Miss., because of dredging.

The problems are not limited to the Mississippi River.

North of Cairo, Ill., the Ohio River has been closed for dredging. That work was also expected to be finished by today.

Earlier this week, the US Geological Survey reported that lakes and streams in more than half of the US had streamflows well below normal for the month of May. According to the survey, the combined flow of the nation's three largest rivers - the Mississippi, the St. Lawrence, and the Columbia - were reported to be an average 37 percent below normal. That is the second-lowest combined flow in 60 years, the survey said.

Dredging is normal on the river this time of year, says David Rich, assistant public affairs officer for the Army Corps' Memphis district. What has alarmed river-watchers this year, however, is that the water levels has fallen so far so soon. At the Memphis gauge, for example, the Mississippi had already reached an all-time record low of -8.6 feet, breaking the -7.3 record set in January 1981.

Normally, the river would fall farther during July and August, Mr. Rich says. But weather forecasters and corps hydrologists now believe that the river can be stablized at its current low levels, he says.

On Wednesday and yesterday, there were thundershowers in the upper Midwest and temperatures cooled significantly, providing some relief. ``We think it's just about to stabilize,'' Rich says. But the river stabilized at low levels doesn't provide much help for barge operators who are already having problems.

``It's devastating,'' says Chris Brinkop, vice-president of river operations for American Commercial Barge Lines Company in Jeffersonville, Ind. ``The [Ohio] river has just never been this low.'' The drought is causing not only costly delays but it is tying up barges that normally would already be handling other loads, he adds.

In Chicago, the Metropolitan Sanitary District has suggested diverting more than the usual amount of Great Lakes water to the Mississippi to raise the water level. But such a move would involve the approval of so many parties, including the US Supreme Court, that it doesn't appear to be a practical solution at all, says Dan Ray, research director for the Center for the Great Lakes.