Concerns that area rivers will flow high in winter and that extreme weather events will result in floods were on the minds of 50 people who gathered Saturday morning at the Yolo Bypass Wildlife Area levee as part of a worldwide demonstration.

That event dovetailed with the recent release of a report that predicts that, locally, climate change will result in more rainfall earlier in the winter and less snowpack runoff later in the season, causing rivers to remain fuller for a longer period of time.

The Davis event was part of "Climate Impacts Day," organized by the international climate campaign organization 350.org, founded by global warming author Bill McKibben.

Events took place at more than 1,000 locations in 150 countries. In the Sacramento region, a similar group gathered at the Guy West Bridge in Sacramento. In Davis, the emphasis was climate change and how it may affect flooding, especially at the Yolo Bypass and levees regionwide.

"We're standing at a very significant space," said Don Saylor, a member of the Yolo County Board of Supervisors.

Saylor motioned to the hundreds of cars racing past on Interstate 80 and the contrast that made with the rice paddies of the Yolo Bypass below the causeway.

It is to that area that water typically is released when reservoirs and rivers grow full during extreme wet weather.

"Flood control issues in the Sacramento Valley have always been a significant aspect here … and as climate patterns change – and there are greater periods of intense rain inundations – those issues will be of great concern," Saylor said.

That concern was mirrored by hydrologist Robert Shibatani, who recently released the report: "Water – Where and When? Planning for Water Supply and Flood Control."

In the report, Shibatani contends that rising temperatures will dramatically change how runoff reaches area rivers, with possible flooding a result.

"The big difference will not be the volume of rain but the timing of it," Shibatani said.

The crux of the report – which models rainfall and runoff patters in the region through 2070 – is that higher temperatures mean rainfall will no longer stay stored as snowpack at higher elevations at places like the Sierra Nevada and lower Cascade Range.

By 2020, Shibatani contends, the snowpack could be as much as 50 percent less than what is seen today.

Instead of water staying as snowpack until late winter, it will flow into reservoirs early. Current state water policy mandates that rising winter water in area reservoirs be released into area rivers in early winter to make room for eventual snowpack runoff.

"We have infrastructure and methods from the 20th century that are currently addressing a 21st-century problem," Shibatani said.

Although peaks in river height are expected to be less severe than those seen during large snowpack melt years, fuller rivers may not be able to absorb prolonged rain events, he said.

"As far as levees go, it will mean that rivers will be full longer," he said. "There will be uncertainty associated with extreme weather events. … You will get an extreme wet year or an extremely dry year."

The flooding issue spurred UC Davis junior Michelle McNicol to don a costume in the shape of a blue raindrop at Saturday's gathering.

"This is my contribution," said McNicol, who is majoring in political science. "I want to bring attention to this issue because, with climate change and increased temperatures, the oceans are rising and we are at risk of flooding."

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