Winter floods swamp the Cosumnes River Preserve, fashioning a vast, temporary wetland that is most attractive to migratory waterfowl. The 6,700-acre Central Valley preserve also entices bird-watchers, who have counted more than 200 species there.
Tundra swans come in for a landing in December and take off before March. Sandhill cranes begin arriving in mid-September, and reach their greatest population (about 3,000) in December. By the end of February, they return to their breeding grounds in Alaska, Oregon and Northern California. Some of the birds that visit the mud flats are short-timers, staying only briefly in the spring on their way to northern breeding grounds and again in autumn while on return flights to wintering areas in Mexico and South America.
The preserve is not just for the birds, however. A 250-acre woodland of valley oaks is said to be the best of what's left in the state. Oaks once covered about 800,000 acres in the valley; they now number about 8,000 acres.
Woodlands and wetlands in these parts have evolved to withstand torrential winter rains and sometimes searing summers. The Cosumnes, anything but a household name to most Californians, is actually the largest undammed river in the Central Valley. Its rain-swollen waters overflow its banks each year, leaving behind a rich silt that nourishes oaks and a variety of flora.
Humans have helped out quite a bit by creating the mud flats so beloved by the birds. Cosumnes River Preserve is a private and public partnership of the Nature Conservancy, Ducks Unlimited, the U.S. Bureau of Land Management, the California Department of Fish and Game and the Sacramento County Parks and Recreation Department.
It was not a natural disaster but a human one that hurt the preserve in December 1995. A fire, deliberately set, authorities believe, destroyed a $1-million state-of-the-art visitor center. Thanks to government and volunteer efforts the center was rebuilt.
Easiest of the preserve's paths is one-mile-long Lost Slough Trail, which incorporates a length of boardwalk to cross the mud flats and allows hikers good views of the abundant water birds. The trail, which makes a square atop low levees to explore the slough, is open 10 a.m. to 4 p.m. daily, and weekends only in summer.
Willow Slough Nature Trail joins a levee to tour both the seasonal wetlands and enduring oak woodland of the preserve. The loop trail is open daily during daylight hours. An interpretive pamphlet, designed by the Nature Conservancy, is available at the trail head and is keyed to 20 numbered stops offering lessons about oaks, hydrology and the willows that gave this slough its name.
Directions to trail head: From Interstate 5, about 20 miles south of Sacramento, take the Twin Cities exit and head east one mile to Franklin Boulevard. Turn south and proceed 1.25 miles to a parking area on the east side of the road and Willow Slough Nature Trail.
(To reach the Lost Slough trail head, continue another quarter-mile past the Willow Slough Nature trail head on Franklin Boulevard to an unsigned parking area on the right.)
The hike: After visiting some interpretive panels near the trail head, join the sometimes very muddy path as it leads a quarter-mile to an oak- and cottonwood-covered lowland. The path extends along a low levee, accompanied by wooden bridges that keep the route above water while crossing the boggy sections of the preserve.
A bit more than a mile out, the path nears the Cosumnes River, reached by several connecting trails from Willow Slough Nature Trail. The main path passes under a railroad overhang, tunnels through riparian thickets, then visits a riverside grove of valley oaks. The path then makes a half-a-mile loop through the preserve's seasonal wetlands before returning to the oak grove. Retrace your steps back to the trail head.
McKinney's book "Day Hiker's Guide to Southern California" is available through The Times for $16.45 (including tax, shipping and handling) by calling (800) 246-4042.
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Willow Slough Nature Trail
WHERE: Cosumnes River Preserve, Sacramento.
DISTANCE: 3 miles round trip
TERRAIN: Wetlands, valley oak woodland.
HIGHLIGHTS: Great bird-watching.
DEGREE OF DIFFICULTY: Easy
PRECAUTIONS: Winter rains can make trails impassable.
FOR MORE INFORMATION: Cosumnes River Preserve, 13501 Franklin Blvd., Galt, CA; 95632; tel. (916) 684-2816