CHICAGO (CBS) — Another sure sign that spring is here to stay: It was spring cleaning day Saturday at Chicago’s Oak Street Beach.

Fifty volunteers with trash bags and clipboards wound their way around people playing volleyball or eating picnic lunches. Sarah Neville of the Alliance for the Great Lakes says the volunteers actually take notes on each item placed into the bags.

Neville said the “research” gathered by the volunteers, analyzed by experts from the Alliance and from Loyola University Chicago, determined that the vast majority of what’s found on Great Lakes beaches was left behind, not washed ashore, although she said one notable exception was a tiki hut somehow found essentially intact, blown ashore near Buffalo, N.Y., during a similar clean-up a year ago.

A volunteer walked up to Neville as she spoke and left a penny found in the sand. Neville said she once found a bag full of $1 bills during a cleanup, which were turned in to police but never claimed, but said the actual amount of change that is found is small.

The Alliance is looking for volunteers who can help with weekly cleanups throughout the spring and summer.

Those who wish to sign up for the weekly clean-up campaign can do so online at www.greatlakesadopt.org.