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RIVERDALE - Two species of birds whose numbers have declined enough to warrant protection under the Endangered Species Act are encountering new troubles in North Dakota.

The rapid rise of Lake Sakakawea has inundated thousands of acres of shoreline rubble that was formerly preferred nesting habitat for the interior Least Tern and piping plover.

While the piping plover is on the nation's list of threatened species, its chance for survival is thought to be better than that of the interior Least Tern where continued concern for their dwindling numbers leaves the shorebird solidly entrenched on the endangered species list.

"That one is afforded protection under the act. It is in imminent danger of extinction," said Mike Morris, U.S. Army Corps of Engineers natural resource specialist stationed at Riverdale.

The Least Tern was first identified by members of the Lewis and Clark Expedition in 1804. It was found in abundant numbers on sandbars and along the shore of the Missouri River. However, man-made alterations to the Missouri have resulted in a massive reduction of suitable Least Tern nesting ground. On the Iowa-Nebraska border alone, sandbar habitat was reduced from more than 35,000 acres in 1890 to just 57 acres by 1976. Fewer than 5,000 adult Least Terns were counted in the most recent population survey.

North Dakota has played a key role in helping stabilize, perhaps even improving, the status of the Least Tern and the piping plover. In 2004, as Lake Sakakawea was beginning a historic drop, six Least Tern nests and 259 piping plover nests were observed. As more shoreline became exposed, the number of Least Tern nests rose to 14 the following year and the number of piping plover nests dropped slightly. Twenty Least Tern nests were counted this year along with 75 nests of piping plovers.

The piping plovers arrive at their preferred nesting locations in late April and into May, often nesting where rising water creates imminent danger. Least Terns don't show up on the nesting areas until late May or early June.

"That actually works in the terns' favor," explained Morris. "The plovers have eggs on the ground in May and can lose their nests to rising water. The birds don't have access to a lake forecast like we do."

To help protect the rare birds, the Corps closely monitors nesting activity. A summer crew actually surveys the entire reach of Lake Sakakawea and the Missouri River from Williston all the way down to Washburn. Nests determined to be at risk of slipping under rising water are flagged and, as a last resort, moved to higher ground.

"Every day our water management office knows where every nest is in relation to the water," said Morris. "The crew that looks for nests of both species enters the locations into the GPS and uploads that data into the system. That provides us with real-time data for every nest on the system."

While flows in the Missouri are often regulated to prevent flooding of piping plover and Least Tern nests, it is not always possible to avoid adversely effecting some nests. Although little can be done to protect nests along the lake against rising water, some known nesting areas have been cordoned off and signage has been put in place to warn visitors to the area of the presence of piping plovers or Least Terns.

"So far just two sites are set aside," said Morris. "One is at Sportsmen's Centennial Park and the other at Wescot, near the Red Mike golf course. Those areas have terns or plover nests or both. People can't go behind the barriers. We are trying to afford some protection and a small buffer for the birds."

A total of 75 piping plover nests were counted along Lake Sakakawea and the Missouri River earlier this year but, even with renesting due to flood waters, the number of plover nests has dipped to 7. Of the 20 Least Tern nests in the original count, 16 remain active.

"We ask anyone along the shorelines to be aware of these species," said Morris. "Show some respect for the birds and give them some room. If you see a broken wing act or are being dived upon, the birds are telling you that you are too close to the nest or the young."