Yukon-Charley Rivers National Preserve

Size: 2.5 million acres. Terrain: Flanked by the Brooks Range and Alaska Range, it often boasts the coldest winters and warmest summers in the country. Alaska's all-time high of 97 was recorded in the area, and winter lows often dip beneath minus 50. Visitors: An estimated 4,942 last year after a hitting a high of 12,784 in 2005. Headquarters: Eagle Getting there: Can't drive - but you can drive the Taylor Highway to the Yukon River, 12 miles upstream of the preserve. Or take the Steese Highway to Circle, 14 miles downsteam of the preserve. Or fly in.

Habitation: Yukon-Charley Rivers may have been one of the earliest homes for humans in the Americas because glaciers did not scour the Yukon corridor after humans crossed the Bering Land Bridge. The Yukon River would have been a natural travel corridor.

Try this: One of America's wildest floats. Many take a charter to the headwaters and float to Circle on the Yukon River, a 75-mile trip that takes six days to the Yukon and two more to Circle. Water is rated between Class II to Class IV rapids on the upper section. Drops 3,300 feet with an average gradient of 31 feet per mile. Rafts recommended over kayaks and canoes due to boulder-laden stretches.

Factoids: North America's largest nesting habitat of American peregrine falcons ... the Yukon Quest International Sled Dog Race passes through Yukon-Charley, with an official dog drop at Slaven's Roadhouse ... The Charley River watershed is the only complete watershed managed by the federal government

Noatak National Preserve

Size: 6.7 million acres

Terrain: One of largest undisturbed watersheds in North America, the Noatak is a 400-mile Wild and Scenic River. Completely enclosed by the Baird and DeLong Mountains.

Visitors: An estimated 2,147 last year, down from a high of 8,383 in 2004.

Headquarters: Kotzebue

Getting there: Can't drive. Fly in by air taxi from Kotzebue or Bettles or travel by watercraft. No facilities of any kind in the preserve.

Habitation: Archaeological and cultural sites date back 11,000 years.

Try this: Noatak offers a spectacular float as long as 280 miles, but beware - the river may still be frozen in June. Mosquitoes are thick in June and July; early September may be best. Be cautious about ice forming on the river by mid to late September. One of Alaska's most spectacular game fish, the aerobic sheefish, is available.

Factoids: Western Arctic caribou herd, North America's largest at more than 300,000 animals, passes through the preserve during their annual treks between wintering areas and northern calving grounds . . . lies entirely north of Arctic Circle . . . home to one of northernmost populations of Dall sheep.

Kobuk Valley National Park

Size: 1.7 million acres in a semi-enclosed bowl 30 miles north of the Arctic Circle

Terrain: Renowned for the 23-square-mile Great Kobuk Sand Dunes, the largest in North America, as well as the Little Kobuk and Hunt River Dunes. Kobuk river on the north side of the dunes winds 61 miles through the park. The park's southern boundary is 35 miles above the Arctic Circle. The boreal forest goes no farther north.

Visitors: An estimated 1,565, down from a record 5,037 in 2005.

Headquarters: Kotzebue. Last July, construction of the park service's 12,350 square foot Northwest Arctic Heritage Center began.

Habitation: Onion Portage is a National Historic Landmark on the Kobuk River where people gathered for 9,000 years to harvest caribou from the world's largest herd as they forded the river.

Try this: Starting in Ambler, a gentle Kobuk River float through the park and ending in Kiana will take about five to seven days.

Factoid: Just one amphibian lives here, the wood frog, which is frozen part of the year but revives in spring.

Bering Land Bridge Preserve

Size: 2.7 million acres

Terrain: The preserve's western boundary is 42 miles from the Bering Strait. Terrain includes lakes, rivers, lava fields and mountains as high as 3,379 feet. Average January lows are minus-15 F on the coast, but winds up to 70 mph produce dangerous wind chills.

Visitors: An estimated 1,019 last year; has declined steadily from 3,740 in 1998

Getting there: Small boats or planes

Headquarters: Nome

Habitation: Some scientists believe the first humans to set foot in North America started here after crossing the Bering Land Bridge.

Try this: There's a first-come, first-served bunkhouse at Serpentine Hot Springs, where you can hike among huge granite tors that encircle the springs, sightsee, fish and relax.

Factoid: Once extinct, re-introduced musk oxen are thriving on the Seward Peninsula ... Devil Lake, the largest maar in the world, is in the preserve ... The westernmost point of the preserve, near Cape Prince of Wales, is 70 miles from eastern Asia.

Aniakchak National Monument and Preserve

Size: 586,000 acres

Terrain: Located some 450 miles southwest of Anchorage, notoriously lousy weather makes it tough to reach. Volcanic activity is possible.

Visitors: An estimated 10 last year in one of the National Park Service's least visited

Getting there: Air charters from King Salmon

Headquarters: King Salmon

Habitation: With no trails, campgrounds or other facilities, there's little sign of human impact here.

Try this: Raft the Aniakchak River from its caldera headwaters at Surprise Lake through treeless tundra to the sea, a three- or four-day trip. It may not be pleasant. One commercial guidebook says: "The weather on Aniakchak is severe; life-threatening conditions can develop rapidly. Extremely violent winds in the caldera, particularly near ‘The Gates,' can shred tents and prevent air rescue." River moves fast through narrow slot in caldera wall.

Factoids: Warm springs drain into the western shores of Surprise Lake in the preserve. Rich in iron sediments, these waters lend a splash of color to the volcanic landscape.

Cape Krusenstern National Monument

Size: 650,000 acres

Terrain: A coastal plain dotted with sizable lagoons and backed by gently rolling limestone hills.

Visitors: An estimated 1,575 last year, down from more than 10,000 in 1985.

Getting there: Fly in by air taxi from Kotzebue. Visitors tend to be skilled backcountry explorers familiar with surviving potential high winds, rain, and snow.

Headquarters: Kotzebue

Habitation: 114 beach ridges, part of a National Historic Landmark, offer evidence of habitation dating back thousands of years.

Try this: Check out the 150 muskoxen who live year-round on the Igichuk Hills of Cape Krusenstern National Monument.