Islands of silt plague Charles

He and others said the massive hump in the waterway, across from the IHOP restaurant in Brighton, is the result of sand, applied to roadways to keep cars from slipping and washed into the river over the decades.

Runoff from a nearby storm drain is also a factor. The sandbar now often rises several inches out of the murky water, depending on the flow allowed through the Watertown Dam.

Boaters have watched the riverbed rise, limiting traffic to a narrow channel, but it became a more urgent concern in recent months after at least two rowing shells slammed into each other, Smith said.

Bob Zimmerman, executive director of the Charles River Watershed Association, compared the problem to what would happen on the Massachusetts Turnpike if traffic were limited to one lane for vehicles traveling in both directions.

“We have a crisis,’’ he said, pointing out that the silt has produced unnatural shallows along the river, most notably the 2 miles from the North Beacon Street Bridge to the Watertown Dam.

“The bottom line is that the storm-water infrastructure we built a century ago doesn’t work,’’ he said. “Something has to be done to fix this.’’

He and Smith have discussed rounding up volunteers to create a bucket brigade to dredge affected portions of the river.

But they realized there could be hazards, given the possibility of arsenic, oil, PCBs, and other pollutants in the muck.

“No one knows what’s in there,’’ Smith said. “We were worried about stirring up the sediment and creating more of a problem.’’

Smith and Zimmerman have begun trying to identify who would be responsible for dredging the river, but they said no local, state, of federal authority has volunteered to do the job, which could cost tens of millions of dollars if the sediment is found to be toxic.

Tim Dugan, a spokesman for the US Army Corps of Engineers in New England, said his agency would probably be responsible for issuing a permit for any dredging. But he said it was not the Corps’ job to dig that far from Boston Harbor, where it is responsible for maintaining sea lanes.