The finding was made by two hydrologists from the University of Saskatchewan who were asked by Environment Minister Dustin Duncan to review operations of the dam at Lake Diefenbaker from August 2010 to the end of July 2011.
The report said the snowpack was underestimated and there weren't enough gauges to monitor flow on the Saskatchewan River system.
"There are deficiencies in estimating inflows and outflows from Lake Diefenbaker that impact on operational decision-making in high-flow events and attempts to reduce downstream flooding," said the 117-page report released Monday.
The report said hydrometric station density is "inadequate" over much of the Saskatchewan River Basin, which is in Alberta and Saskatchewan. It said the situation is particularly acute in the Saskatchewan portion of the basin.
"The lack of a station downstream of the confluence of the North and South Saskatchewan rivers makes assessing the impact of releases from Gardiner Dam on the Saskatchewan River extremely uncertain and is considered to be an extraordinary omission," it said.
"The paucity of stations for the Saskatchewan portion of the Lake Diefenbaker drainage make this region almost entirely ungauged, which resulted in the inability to measure April inflows into the lake in 2011."
The high flow of 2011 and flooding in the region started in the summer of 2010 with wet conditions on the Prairies.
Precipitation was 200 per cent of normal immediately upstream of Lake Diefenbaker in the summer of 2010. More rain that fall led to soil at or near saturation at the time of freezeup, which meant the ground couldn't absorb the subsequent spring runoff.
Mountain snowpacks were also higher than average by spring 2011.
Lake Diefenbaker receives inflows primarily from the Rocky Mountains and a very small proportion from Saskatchewan.
The report also noted that the Gardiner Dam is one of the few major control structures in North America operated for multiple purposes without a formal ranking of priorities. That means there are no rules about what comes first: flood control, hydroelectric generation, water supply or recreation.
"While there was a clear concern to assess risks in mid-winter, we could not find a formal ranking or list of priorities for risk assessment; it would appear that the dam is managed with an attempt to minimize risk of failure of any of the priorities for the dam," said the report.
"Certainly the email correspondence shows attempts by interest groups to influence the priority for risk management 'on the fly.'"
The report said priorities are needed "instead of the very unclear and ad hoc operating regime" currently in place.
"If water supply concerns are paramount, then the higher minimum lake levels can be sustained, but the public must be informed of the changing flood risk associated with this operational decision."
The report said watershed authority staff did a good job with the limited resources.
Duncan acknowledged that a new operating plan is one of the key recommendations.
"There are a number of stakeholders around Lake Diefenbaker and Gardiner Dam that have different expectations of the dam," said Duncan.
"But as the report states, there is no clear prioritization of basically who has more priority over the water than anybody else, so we get into a situation like we did this year where we have significant flooding, which means we need significant releases from the dam.
"And people have an expectation that they are the top priority, when in fact we really don't have a system that does prioritize it."
Duncan would not say what he thinks the priority should be for the dam. He said the watershed authority will be consulting with stakeholders to develop a new plan, but it won't be ready for this spring's runoff.
"I don't want to leave the impression today though that those changes will drastically alter people's use of the lake right now. But we do need to have a clear operating plan that clearly identifies what the priorities are and what the priorities are depending on the situation."