The Trump administration argued parkgoers should decide for themselves how to stay hydrated.
But the new report, published because of a Freedom of Information Act request, shows the policy did some good. The report estimates the rule prevented between 1.3 million and 2 million bottles from being sold and used every year.
And it estimates the policy prevented CO2 emissions by up to 141 metric tons each year.
The National Park Service distanced itself from the report, saying it was originally meant for management to make decisions on the policy and that NPS lacked the data to ensure the findings.
Still, Rep. Mike Quigley of Illinois wants the administration to reinstate the ban and make it permanent.
Quigley argues the report shows "that the Trump Administration will continue to deny science, research, and facts in its efforts to prioritize big corporations at the expense of our wildlife and environment."