Muscles become easily fatigued in cold water, which can lead to trouble while enjoying local waterways.

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SACRAMENTO – Memorial Day is the unofficial start of summer.

Some people will flock to local waterways to cool down as the summer heat rolls into the area. But, swimmers and boaters may be surprised to find the water cooler than expected.

Right now, the average temperature in the Sacramento River is 58 degrees. Even though the air temperatures may reach the 90s, the much cooler water temperatures could be a hazard.

"The body will react by shivering and the longer you stay in the water, your core temperature will drop," Sacramento County Chief Park Ranger John Havicon said.

With the average body at 98.6 degrees, jumping into water about 40 degrees cooler can shock the body. According to Havicon, your muscles and movements slow-down in the frigid water.

"Your muscles won't work as well and it could be difficult to get yourself out of a situation," he explained

Water takes longer to heat than air because of thermal conductivity. The simple definition of thermal conductivity is a measure of how a material transfers heat. The ground has greater thermal conductivity -- because the earth is dark and less reflective, it absorbs the sun's radiation. Water, on the other hand, will reflect more of the sun's radiation and heat.

If you are planning on taking a dip in a river or lake over the holiday weekend, Havicon strongly suggests you wear a life jacket. The California Division of Boating and Waterways, DBW, has a life jacket loan program. You can check out a life jacket at various locations along the state's waterways.

You can find a location nearest you at http://dbw.parks.ca.gov/BoaterInfo/LifeJacket.aspx

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