Dry conditions over much of the Bay watershed helped to make 2015 the fourth-best year for overall Bay water quality in three decades, according to figures released Wednesday by the Chesapeake Bay Program.

Monitoring data from the state-federal partnership showed that 37 percent of the Bay and its tidal tributaries had adequate oxygen, water clarity and the reduced algae concentrations needed to meet water quality goals for the three-year period of 2013–15.

That was a 10 percent improvement over the previous 2012–14 assessment period, though it was still well below the 100 percent attainment needed to meet Bay cleanup requirements. Goal attainment is measured on a three-year rolling average.

Much of the improvement stemmed from reduced rainfall — river flows into the Bay were about 25 percent less than average last year, officials said. Dry conditions mean fewer water-fouling nutrients are washed off the land and into the Bay, where they spur algae blooms, as well as less water-clouding sediment.

Bay Program figures show that the amount of nitrogen reaching the Bay last year decreased 25 percent from 2014 levels, while phosphorus decreased 44 percent and sediment 59 percent.

The years with higher marks for water quality — 2001, 2002 and 2010 — were even drier. In those years, slightly more than 40 percent of the Bay met cleanup goals for dissolved oxygen, water clarity and chlorophyl a, which is a measure of algae concentrations.

Officials insisted that last year’s improvements were not just due to weather, as long-term trends show nutrient trends are decreasing in many places.

“A lot of what we saw this particular year, this large drop in [nutrient and sediment] loadings to the Bay, was due to low river flows,” said Scott Phillips, Chesapeake Bay Coordinator for the U.S. Geological Survey. “But we are seeing a long-term benefit of the management actions, also.”

Nutrient discharges from wastewater treatment plants — which collectively have already met 2025 cleanup goals — and  nitrogen emissions from tailpipes and smokestacks have been sharply reduced, they said, while farmers have ramped up their implementation of conservation practices to reduce runoff.

As a result, 2015 water quality was better than it was during several past years with similar river flow amounts into the Chesapeake, said Rich Batiuk, associate director for science with the U.S. Environmental Protection Agency’s Bay Program Office.

“Even for equal amounts of rain 10, 20 years ago, we are actually seeing less pollution coming into the Chesapeake,” he said. “Are we done? Absolutely not.”

Indeed, computer models — which estimate the effect of nutrient reduction actions taken on the watershed under average weather conditions — show the region is not on pace to achieve the nitrogen or sediment reductions needed to fully meet its water quality goals.

In a statement, Kim Coble, vice president of the Chesapeake Bay Foundation, said the water quality improvement was good news, but “weather played a key role and that is something we can’t count on every year.”

She called for greater efforts to get nutrient reduction efforts on track in Pennsylvania, where pollution control efforts are lagging badly.

Still, officials are relishing the  improving Bay trends in recent years. Clearer water contributed to the expansion of underwater grass beds in the Bay last year — which are at a 30-year high — and that expansion may have aided the comeback of blue crabs.

And, they said, anecdotal reports continue to come in of people who are seeing clearer water than they have in years, and of continued recovery of underwater grass beds, an important habitat for many species.

“I can attest personally,” said Nick DiPasquale, director of the EPA Bay Program Office. “I was on the Gunpowder River kayaking two weeks ago. The underwater grasses there were so thick you could almost walk on them.”