BALTIMORE (WJZ) — There’s been progress but there’s still a lot of hard work to do. That’s the message Maryland legislators received in a briefing on the state of the Chesapeake Bay.

Alex DeMetrick reports the hardest problems are all our hard surfaces.

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From the day it went up, Conowingo Dam has been trapping sediment washed off the land in storms. For every one million pounds of sediment that goes over, the dam holds back two million pounds. But that reservoir of liquid dirt behind the dam is filling up.

“One million goes over. Once it reaches equilibrium, all three go over. Where do they go? Maryland,” said Ann Swanson, Chesapeake Bay Commission.

That was just one of the big worries presented to state legislators by environmental groups. Within 10 years, huge amounts of sediment could flow into the bay from Conowingo. That sediment would wreck water clarity, blocking sunlight and smothering underwater plants. It’s a setback just as the bay is showing signs of reviving, not just in the growth of fish and crab populations, but in the reduction of nutrients like nitrogen, which feed algae that cause dead zones.

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That’s a direct payoff from improving sewage treatment plants.

But the hard surfaces of roads, parking lots and sidewalks remain a major obstacle. As development grows, nature’s ability to clean water shrinks.

“We are losing our natural filters to hard surfaces. We aren’t going to see improvement in the urban/suburban sector without taking a hard look at how we grow and where we grow,” said Alison Prost, Chesapeake Bay Foundation.

Growth responsible for a quarter of all pollutants still pouring into the bay.

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The cost for dealing with runoff from hard surfaces is estimated in the billions. As for Conowingo, dredging sediment from behind the dam would cost $60 million a year.