A subtle but expensive form of vandalism is spreading through the popular mountain-bike trails along the Kansas River.

In recent weeks, unidentified motorcyclists have caused nearly $19,000 in damage to the winding, hilly trails — not by tearing up the existing trails but by forging new ones.

Neil Taylor, trail coordinator for the Lawrence Mountain Bike Club, describes the problem of motorcyclists riding on bicycle trails along the Kansas River levee. In recent days motorcycles caused nearly 9,000 damage to the trails.

Aside from the fact that motorized vehicles are banned from the park, one of the biggest concerns is trail users’ safety, said Neil Taylor, trail coordinator for the Lawrence Mountain Bike Club. New trails make it more likely bicyclists will stray from the main loop and go the wrong direction or collide with another cyclist where the trails cross.

“We have families and kids and moms and dads,” said Taylor, one of 60 club members who volunteer to maintain the trail. “People can get off on these trails, get confused and wind up going the wrong way.”

The unwanted trails, which have spread for about three miles, also cause soil erosion, Taylor said. He said club members likely would volunteer their time to regrade, pack and seed the soil in the fall, when environmental conditions are best.

The most recent problems began about two months ago. A few weeks ago club members installed “No Motorized Vehicles” signs, but that hasn’t solved the problem.

The most recent vandalism happened between midnight and 9 a.m. April 9, according to a police report released Monday. There were no witnesses April 9, but earlier in the month a cyclist saw people riding what appeared to be dirt-bike motorcycles in the area, Taylor said.

6News video: Motorcycles damage river trails

He’s hoping someone eventually will spot a license plate number and report it to police if the problem continues.

“It’s really frustrating to us because what we’ve done basically gets torn up,” he said.