FOR TODAY'S LESSON in governmental confusion, consider the Army Corps of Engineer's approach to water-supply planning in this area. The Corps' Washington Aqueduct division supplies Potomac water to the District, Arlington and Falls Church. The Corps has veto power over a new intake that the Washington Suburban Sanitary Commission wants and any Potomac pipelines that Fairfax County may propose. Obviously, if the Potomac basin is hit by drought, the river may not be able to meet all these demands. Thus the Corps persuaded Congress last year that the WSSC project should be deferred until all authorities had agreed on how to parcel out the water when the river's flow is low.

That's sensible. But now everyone else has reached agreement - and the Corps won't sign. Why not? The engineers say that the pact would not insure enough clean water for the aqueduct system. They also think longer-run supply problems should be solved first. In essence, they won't agree on how to cope with a shortage because there might not be enough water to go around.

It is a real catch-basin 22 approach. The Corps likes catch basins, of course. The engineers are now building the Bloomington reservoir upstream and restudying the controversial Verona Dam proposal in Virginia. And they continue to insist that more impoundments are the best insurance against future water crises here.

At Congress' direction, the Corps is also starting a major study of the Potomac estuary, the biggest and most convenient catch basin of them all. If estuary water can be rendered fit to drink, most of the region's long-term water worries will evaporate. However, the study is not scheduled to be finished until 1982. Meanwhile, the engineers are not enthusiastic about drawing even small amounts from the upper estuary in emergencies - though other experts think that this would be quite safe.

And so it goes. The current incoherence of water policies here should not be blamed entirely on the Corps; as recent articles by staff writer Douglas B. Feaver made clear, every authority involved has contributed to the hassling and hand-wringing that have frustrated all attempts to shape a regional strategy. The situation probably can't be straightened out, however, as long as the Corps has so many diverse assignments as water supplier, overseer, dam-builder and analyst. If President Carter wants to tackle another tough reorganization problem, he should take a look at this.