Yellowstone National Park officials did not fully examine all aspects of winter-use impacts in the park and will delay implementing a final winter plan while more studies are conducted, Superintendent Dan Wenk said Thursday.

While those studies go on, temporary guidelines allowing as many as 318 snowmobiles and 78 snowcoaches per day in the park will remain in effect for the coming winter, he said.

“This has been a very long, arduous process that’s been expensive. And that’s part of my rationale for trying to make sure that we get this right," Superintendent Wenk said during a media conference call. "We could have made a decision to try to plow it ahead and try to fix the issues that came up in public comment. I didn’t believe it was the most prudent way to go.

"I believe the way that we’re proceeding now recognizes the questions, or honors the process of public comment in bringing issues to our attention, and I think it’s the most efficient, most cost-effective way to move forward to try to bring this to a conclusion that will have a sustainable result," the superintendent said.

For more than a decade, at a cost "in excess of $10 million," Yellowstone officials and staff have been trying to develop a feasible winter-use plan that suits both those in favor of recreational snowmobile use in the park and those who believe snowmobiles should be blocked from entering the park. Lawsuits, threats of them, and politics time and again have forced the park staff back to the drawing board.

The latest effort produced a Draft Envionmental Impact Statement of more than 550 pages with seven alternatives ranging from no over-snow visitor traffic in the winter to a variety of mixes of oversnow traffic. But problems with the underpinnings of those alternatives led the superintendent to call for more studies on items ranging from air quality and soundscapes to "best availability technology" requirements for snowcoaches and continued access over Sylvan Pass.

"The public comment period did exactly what we hope public comment will do in a process like this, in that it identified issues that were not addressed as fully as they should be perhaps, or brought to our attention issues that we did not consider," he said.

"Bottom-line, all those things point to the fact that we are not able to get a long-term decision in place prior to the the '11 and '12 winter-use season," said Superintendent Wenk.

The new goal, he said, is to get a long-term plan ready to go for the 2012-13 season.

Among the unresolved questions, Superintendent Wenk said:

* He wanted to know how noise from snowmobiles and snowcoaches actually occurs. "What I’m asking is, is there a better way to look at the size of groups, how many snowmobiles should be allowed per group that enters, should we have snowmobiles travel in groups, or should we have them individually?" he said.

* More work is needed to determine the consequences of keeping Sylvan Pass safely open for over-snow traffic. “There were a lot of questions raised, and we need to do a further analysis of the impacts that are from the actions that we take on Sylvan Pass in terms of to bring down the avalanches," he said. "There were a lot of questions raised about the impacts on wildlife by those actions that we think we need to do further analysis. Whether that will change the outcome or not, I’m not making a predetermination on. But we believe that we have to have more analysis.”

* Park staff did not consider impacts on air quality and soundscapes using 2010 fleet data. "We did not use the right modeling, and so we are going to go back and fix that," the superintendent said.

* Park staff is uncertain whether sound levels prescribed for "best available technology" snowcoaches are achievable.

* The DEIS did not examine the impacts of allowing non-commercially guided snowmobile trips.

The park received comments from nearly 59,000 individuals, and within those there were roughly 179,000 specific comments concerning winter-use in Yellowstone, officials said. Of those, 82,362 specifically called for snowcoach-only travel in the park, they said.