BILLINGS - Noise emissions at three key spots in Yellowstone National Park were "substantially lower" last winter, but still routinely exceeded thresholds set by the National Park Service, a new study shows.
Despite lower snowmobile numbers and requirements that the machines be cleaner and quieter, researchers in the park said noise continued to be an issue at three monitoring sites: near Old Faithful, at the West Yellowstone entrance and at Madison Junction.
Researchers tallied about 1,500 instances at the three monitoring sites in which noise levels exceeded the Park Service's threshold of 70 decibels. The majority of the cases involved snowmobiles, although snowcoaches also exceeded limits, but not as frequently. Seventy-seven instances were attributed to a snow grooming machine.
"One thing that's clear is there's further improvements to be made," said Mike Yochim, an outdoor recreation planner at Yellowstone.
The monitoring report is the latest in a series of reports that have been released in recent weeks looking at the effects of snowmobile and snowcoach travel in the park last year.
The data is expected to be used as the National Park Service and Department of Interior draw up a long-term winter travel plan for Yellowstone and Grand Teton national parks.
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That plan is the third in-depth study of winter use in the park. While it's being drafted, temporary rules remain in place that allow up to 720 snowmobiles a day in Yellowstone. The machines have to meet "cleaner and quieter" standards and riders must be accompanied by commercial guides.
Noise and air pollution and their effects on wildlife and habitat have been key issues in the debate over motorized use in the park. The Park Service has said that preserving natural quiet is an important part of its mission.
With the results of the latest study, Yochim said the Park Service will look at whether it's technologically feasible to make snowmobiles quieter in Yellowstone.
"We hope to have a discussion with (snowmobile) manufacturers as to their take on that," he said.
Ed Klim, president of the International Snowmobile Manufacturers Association, said Tuesday that he had not seen the study but would question several aspects, including how researchers determined what vehicles were making the noise.
"I know there are quite a few snowcoaches that run around the park and there weren't a lot of snowmobiles last year," he said.
A switch from two-stroke snowmobiles to four-stroke machines, through a Park Service requirement that machines meet best available technology standards, has cut noise levels in Yellowstone in recent winters.
Park officials wanted to know if the daily limits for snowmobiles, along with the technology requirements for the machines, had an effect on noise in the park.
Researchers monitored sound between December 2004 and March 2005 at Old Faithful, the West Entrance, roadways and two backcountry areas.
Reductions in overall noise, as compared with the previous winter, were mainly attributed to fewer snowmobiles in the park. Snowmobile traffic averaged only about 200 machines per day.
The report made several recommendations, including continued sound monitoring, reducing the noise of "oversnow" vehicles and increasing the number of locations where samples are taken.