LOS ANGELES, SEPT. 3 -- A drought described by some experts as the worst in a decade has dried out much of the West and helped start more than 1,815 brush and forest fires now raging in six states.

Continued high temperatures and low humidity, particularly in the usually soggy Pacific Northwest, are expected to keep many fires out of control at least through the weekend. More than 18,000 firefighters, some from as far away as Maine and Alaska, are trying to put out the lightning-sparked blazes.

The drought has become particularly serious in Oregon, where an unusually dry winter has left some streams and rivers even lower than the last major drought in 1977. Beverly J. Hayes of the Oregon Water Resources Department reported that some reservoirs are empty and others, such as Cottage Grove and Dorena, are so low that they can no longer be used for boating.

"We have to have some more rainfall," Hayes said, or more drastic conservation measures will be necessary. Her office issued an appeal for voluntary water-saving measures today. Residents of the town of Powers, in a particularly parched area of southern Oregon, are about to be ordered to stop watering lawns.

Bob Potter, spokesman for the California Department of Water Resources, said the state suffered a particularly dry winter. A month ago, he noted, Water Commission Chairman Clair Hill said hillside grass and brush near his home in Redding looked drier and more withered than at any time since the great western drought of 1976 and 1977.

A check of state records, Potter said, revealed little California rainfall since 1984, except for a freak storm from the south in February 1986 that flooded large areas of the Central Valley and helped refill some reservoirs.

Except for a few towns that rely on local streams, California has no serious water shortage now. But "that won't be true if the next winter is as dry as the last one," Potter said, "and if the winter after that is also dry we will really be in trouble."

Jay Malinowski, spokesman for the Metropolitan Water District of southern California, said reservoirs are at about 60 percent of capacity, somewhat below average. Water users are being encouraged to conserve, he said, but Los Angeles has little to worry about immediately. The heavy Rocky Mountain snows and runoff of 1983 to 1985 left the major Colorado River reservoirs, such as Lake Meade, full. It would take much more than a year of drought to drain them to a critical level, he said.

But Oregon and some parts of northern California, he said, are less equipped for sudden droughts because rain is usually so plentiful. "In Oregon, where people rust, there is no point in spending $2 billion to build a huge dam and reservoir. If you expect to get flooded out 16 years out of 17, that 17th year when you have a drought, you just bite the bullet."

Michael Cowan, chief of the water operations branch for the U.S. Bureau of Reclamation in Sacramento, confirmed that reservoirs in his area are 20 to 25 percent below average levels but nowhere near a significant shortage. He said that although the dry winter may have made grass and brush more combustible, it also retarded their growth and may have limited the severity of some fires.

Most of the fires are burning inside a broad band from the foothills of Mount Palomar through the Mojave Desert, the Sierra Nevada and the Cascades. Authorities in Arizona, California, Oregon, Idaho, Montana and Wyoming reported nearly 400,000 acres burned or burning since early August. The reported amount of blackened acreage in northern California, the worst affected area, jumped from 141,243 acres Wednesday evening to 296,846 this morning, California Forestry Department spokeswoman Robyn Lawton said.

Dana Bornheim, a fire information officer brought from Colorado to help at the Interagency Fire Center in Boise, said that "for size and intensity and threat to life and property and natural resources, this is probably the worst we've seen in the last decade." So far, however, there has been only one reported death -- a firefighter at Klamath National Forest in California who was struck by a motorcycle.

About 12 residences in California and 10 in Oregon were reported destroyed and more than 5,000 people in California and Oregon have been advised to evacuate, with large numbers so far not complying. Forest fighting resources were stretched so thin, with every single air tanker in use, that some remote blazes were left to burn themselves out. A fire in the mountains of central Idaho has been burning for 33 days and consumed 17,500 acres, but U.S. Forest Service officials consider it a natural phenomenon that does not require containment.

Fire officials said that more than 900 members of the Army and Air National Guard, plus some prisoners, were helping assist firefighter crews stretched to the limit. Officials in California expressed fear that one blaze might endanger some giant sequoias.

Officials at the Boise headquarters Wednesday could see smoke drifting over the Idaho capital from fires to the west. In southern Oregon's Rogue River Valley the smoke became so dense that people with breathing problems were urged to stay indoors.

William Shenk, the forest service's fire equipment branch chief, called the sudden rash of fires in the past week a freak of nature caused by a series of electrical storms sweeping the Northwest. The storms brought lightning and wind, which ignited and fanned flames, but no rain to put them out. Usually, he said, lightning storms -- the principal cause of forest fires -- space themselves throughout the summer, giving firefighters a chance to contain one blaze before moving to another.

The dry winter, he said, made things worse. Shenk now works in Rosslyn, Va., but spent more than three decades in Oregon and Washington. "Usually it would level out. If you had a dry winter, you'd have a wet summer, but not this time," he said.