For the second year in a row, volunteers will gather off Washington Street to dig through the mud flats at Mill Pond.

The volunteers will be taking part in the second annual survey this morning and tomorrow, from 9 a.m. to 2 p.m. Access to the site is through the backyard at 375 Washington St.

Last year, for the first time in years, clams and worms were discovered growing in Mill Pond — the old and over-engineered economic estuary that drains the central watershed of Cape Ann, in the 17th century, provided the first water power for milling corn.

Salem Sound Coastwatch is conducting this survey in conjunction with the city of Gloucester's effort to restore the natural tidal flow to Mill River. The purpose of the study is to quantify changes in the living organisms in the benthic (mud) layer since the tide gate was opened at Mill Pond during the Mothers Day floods of 2006.

Coastwatch is part of a coalition of organizations working to restore the pond to something like its virtually prehistoric conditions, when it was the Cape's only major estuary in which large volumes of fresh water flowed into a tidal basin. The flow to Mill Pond emptied from the Babson watershed, the largest on the island. The great alteration of that natural environment was the construction of the Babson Reservoir Dam in 1930, but 17th- and 18th-century drawings show multiple dams of Mill Brook along what is now Poplar Street.

A number of volunteers will work at the edge of the mud flats at low tide today and tomorrow. Working in teams, they will dig buckets of mud samples from designated areas; carrying the mud to a screening station; washing the mud through screens to capture the clams, worms and snails left behind; and counting, measuring and recording all of the living organisms.

The first survey took place last October. Findings showed there has been a return of benthic marine species, including soft shell clams, since the tide gate was opened.

"The initial findings are encouraging," said Barbara Warren, executive director of Salem Sound Coastwatch. "However, community recovery in restored estuarine ecosystems depends largely on the level of tidal exchange. When tidal flow is unrestricted, the estuary can return relatively quickly. But when the area is not fully flushed with sea water, it may never fully recover without additional hydrologic modifications."

This project is supported by technical and financial assistance from the Massachusetts Wetlands Restoration Program, the NOAA Restoration Center/ Restore America's Estuaries partnership, and The Massachusetts Corporate Wetlands Restoration Partnership (CWR) through the Bruce J. Anderson Foundation and Metcalf Eddy Inc.