Debris is of concern in the Grand River, which empties into Lake Michigan at Grand Haven, as well as other waterways. Boaters in the Grand Haven and Holland areas are among those being urged to use caution.
"We have seen and we still continue to see trees and logs of a significant size floating down, and we do get a lot of reeds and grass and things of that nature that come down," said First Class Petty Officer Kent Sypniewski. Boaters should stay "highly alert."
It's not known whether the flooding was responsible for all of the debris.
The flooding began in April and lasted through early May, hitting West Michigan and some other parts of the state. Floodwaters from the rain-swollen Grand River poured into Lake Michigan, carrying debris including trees and even the remains of boat docks.
"Every time there's someone on the water, they should be keeping an eye out because they never know what's floating just beneath the surface," he said.
In addition to large debris, some areas along riverbanks and shorelines have seen smaller debris build up because of the flooding. Jim Kearns, a boating safety instructor, said there have been reports of boats being damaged by uprooted or fallen trees in the river.
"I have been cautioning boaters to slow down and add another lookout," Kearns wrote in an email.
The "Grand River GreenUp" takes place Saturday, hosted by the Grand Haven Jaycees and West Michigan Environmental Action Council, and aims to tackle some of that debris. About 20 miles of the Grand River, adjacent tributaries and the Lake Michigan shoreline will be combed for trash.
The GreenUp is in its third year. It initially had been scheduled for before April's flooding, but was postponed because of safety and other concerns.
"It's a very timely event this year," event chair Brock Rodgers said in a statement. "Had we cleaned up before the flood, a lot of work might have been literally washed away. New trash would have been deposited there from upstream."