They could float, but they couldn't hide.

Not if GW Mechanical Inc. could help it.

A crew from the Mills company spent Saturday morning building an aquatic corral at Crossroads Park to gather thousands of rubber ducks for their sojourn later that day after they were dumped in the drink at Jonah Bank on First Street for the Great Duck Derby during the sixth annual Platte River Parkway Festival.

For weeks, people have been buying individually numbered yellow rubber ducks for $20 each.

By Saturday morning, nearly 5,000 had been purchased by people wanting to support the Casper Rotary Club and in the hope that their little yellow friends would float the fastest and win them cash or one of 200 prizes donated by local merchants, club president Bill Sullivan said.

But the ducks needed somewhere to go, and that's where GW Mechanical came in.

By the time its crew arrived, parallel lines of green, yellow and pink foam rods formed a channel in the river from the bank to the park.

In years past, some of those rascally plastic waterfowl jumped the channel and Duck Derby volunteers waded into the stream with nets to snare the escapees.

Even then, a few eluded their would-be captors to float free and make a run for Glenrock.

Not this year.

A boom of red and gray floats stretched across the river 35 to 50 yards downstream to thwart escape for all but the most determined David Quackerfield.

With that infrastructure in place, GW Mechanical's employees passed tools back and forth to their coworkers standing in thigh-deep water.

They positioned a board to further narrow the channel and guide the ducks to a long, narrow wooden chute where the dutiful drakes would form a single-file line for their ultimate entrapment in a plastic tube.

The numbers of the first 200 ducks would be tallied in a nearby RV where the winners were determined.

To contain the losers, Brandon Cockrum pounded rebar into the riverbed to anchor the pens and a blue tarp.

Mike Kruchek said GW Mechanical has been setting up the corral for about five years.

The efforts of this company and the hundreds of other volunteers help the Casper Rotary Club support club projects such as Crossroads and Rotary parks, and a dozen local nonprofit agencies such as the Boys and Girls Club and 4-H, Sullivan said.

Last year, the club bought 10 "shelter box tents" that come with enough living space, tools, toys, sleeping bags, water purification systems, stoves and utensils to help a family survive for up to six months, he said.

Those tents went to Sri Lanka, and the club wants to buy more this year, Sullivan said. "It's something Rotary Clubs have done all over the world."

The club has arrangements with charities that will sell the ducks for $20 and keep $10 for their own projects, he said. "They make very good money that way."

Reach Tom Morton at (307) 266-0592, or at tom.morton@trib.com. Read his blog at tribtown.trib.com/TomMorton/blog.