A sure sign of spring is when the ice is gone from Lake Minnetonka, the metro area's largest lake.

That happened Sunday morning, when the Hennepin County Sheriff's Water Patrol took a spin on the lake and declared that boats can navigate through all of its channels and bays. The ice-out notice for 2015 is 19 days earlier than 2014, and nine days earlier than the median ice-out date of April 14.

The Water Patrol had help from the Freshwater Society, whose members monitored ice from shore and accompanied deputies on daily boat trips recently to check areas where ice remained.

Sheriff's deputies Jeremy Gunia and Shawn Eberle made the announcement Sunday morning with Kent Warden, Freshwater Society board member.

Ice-out records on Lake Minnetonka date back to 1855, and officials say they are a valuable if inexact method for revealing long-term trends.

The earliest ice-out date for Minnetonka was March 11, 1878, and the latest was May 5, 1887.

It's not an easy task to determine when the ice is gone, since Lake Minnetonka has 37 bays and is just over 14,500 acres in size. The definition of ice-out on the lake means that it's possible to boat through all the bays and channels without blockage, although some thin ice may remain in some areas.

An estimated 10,000 boats dock on the lake and 61,000 boats use its launches.

Last year the ice-out date for Minnetonka was April 24, but the boating season was hampered by unusually high water from spring rains and the wettest June in Minnesota history. The lake reached record high levels and the Lake Minnetonka Conservation District imposed unprecedented no-wake speed restrictions for the entire lake that lasted until late July.

This year it's the opposite problem, with water levels lower than normal in many lakes. As of Friday, the Twin Cities area was in its ninth-driest year in 145 years, with 92 percent of the state in a moderate drought, according to the National Weather Service.

Although Minnetonka gets plenty of attention as the state's most popular lake, many smaller lakes in the metro area and throughout southern Minnesota had ice-out dates this year as early as mid-March.

Different lakes have different methods of determining ice-out, but the trends are important as long as they're consistent from year to year.

Go to dnr.state.mn.us/ice_out for a map of other ice-free lakes across Minnesota.