The White Mountains, which spread across northern New Hampshire and into Maine, form the most mountainous region in New England. The breathtaking scenery , the ice-cold mountain streams and waterfalls, and the many picnic spots and campgrounds make this a delightful and much-used outdoor recreational area.

When my husband and I go there, we tend to shy away from the more popular spots, concentrating instead on the less-visited northern section of the mountains. We had always, for instance, avoided the Kancamagus Highway - considered by many the most scenic road in New England - despite the allure of its numerous campgrounds (which are always full in the summer) and hiking trails (again, which tend to be used by many people).

During the second week in October, the ''official'' leaf-watching week, the Kancamagus is an extremely busy highway. The undulating mountain road offers spectacular views of brilliant red maples and other mountain foliage.

One recent morning, we decided to change our habits and take a pre-foliage drive along the renowned highway. It was a good decision. The towering pine trees, sparkling river, almost overpowering mountains, and shimmering birch trees fascinated us. The early morning fog, swirling between the trees and across the river, gave the drive a mysterious, ethereal quality.

Our first stop was the Boulder Loop trail, accessible from the Covered Bridge Campground. We crossed the Swift River over the well-cared-for covered bridge and found the sign for the trail, a 2.8-mile hike described in the guidebook as ''a gradual climb with some steep pitches.'' (For any person accustomed to rough walking this short hike is a pleasant hour-and-a-half.)

The trail is indeed a gradual climb, but it traverses along sheer outcrops of granite that look as if they have been carefully placed to form an impassable wall. It was dark along the trail because the towering trees blocked out what little sun there was. The view from the top of the trail is magnificent - sheer cliffs, the rushing, winding river, and distant tree-covered mountains.

The return hike was equally beautiful. Dense green underbrush, fallen trees, and a muddy, leaf-filled stream outline the gently sloping trail. Halfway down it started to rain, lightly but persistently; before we got to the end of the trail, it was coming down quite hard. I ran the last part and reached the car just as the real downpour began.

After a breath-catching rest in the steamy, hot car, we decided to continue our drive up the highway. We planned to scout out the sights and then stop at them on the return trip. As we drove along, passing Lower Falls, Rocky Gorge, Passaconaway, Champney Falls, and Sabbaday Falls, the rain lessened. By the time we had reached the end of the 30-mile drive the clouds had cleared and the rain had stopped.

We turned around and drove to Sabbaday Falls, a very short, picturesque hike. Sabbaday Falls, named by the early New Hampshire settlers who stopped here for water on the Sabbath, is a series of waterfalls cascading into a narrow gorge, forming a crystal-clear pool. Wooden stairs and bridges enable the hiker to go to the top of the falls without difficulty, and carved plaques tell the history of its formation.

The Chapney Falls trail, one of the trails to the top of Mt. Chocorua, is longer, somewhat steeper, and not as clearly defined as the Sabbaday Falls trail. We had to keep a sharp lookout for the yellow trail markers to be sure we were going in the right direction.

The people we met on the hike were friendly. We exchanged the traditional ''Hello, how's the trail?'' and, without pausing, continued the climb, each hiking party respecting the other's privacy.

Gnarled tree roots and soft pine needles carpet the winding trail, and the sound of water deadened the noise of our feet - even though the East's dry summer has reduced Champney Falls to a relative trickle. The lack of water allowed us to climb to a large boulder in the middle of the stream bed. We sat there, the noises of the forest silenced by the falls, looking down the stream. No one else was within sight or earshot. We seemed to be in another world of solitude and peace.

Just below the Champney Falls are the Pitcher Falls, a narrow cascade of water, plunging over a steep cliff. The moss-covered granite walls rise sharply on either side of a ravine, and the falling water forms a large pond at the bottom.

The Lower Falls, along the Swift River, is the largest and most popular picnic area along the Kancamagus. The rapidly cascading water and clear, deep-blue pools look cool and inviting. The large boulders, which are easily accessible from the river bank, provide warm, but not so comfortable, sitting places for picnics and sunbathing.

We found a very large, flat, sun-warmed boulder on the edge of the river. We had brought a real picnic - a basket with plates and silverware, and a cooler with fruit salad, vegetable salad, cheese biscuits, and cold chicken.

We watched the people up river take short swims and float around in inner tubes. The serenity and solitude of the mountains was palpable even at this crowded swimming area where strangers said, ''Hi,'' and helped each other across the stream. Practical Information

The most complete source of information is the Appalachian Mountain Club White Mountain Guide

The guide lists every trail and includes a detailed description of important markers, the steep sections of the trail, the distance, and the time it will take the average walker to complete. The guide is accurate, and the time allowed for completing the hike generous. The AMC guide, which costs $9.95, is available from the Appalachian Mountain Club, 5 Joy Street, Boston, Mass., or through a bookstore.

The traditional week for fall foliage touring is the second week in October. During this week the traffic is very heavy. It is best to begin touring before 9 a.m., when traffic is a little lighter. The weekdays are less congested than the weekends and Saturday is less crowded than Sunday.

The American Automobile Association gives fall foliage advisories beginning the second or third week in September. AAA tracks the changing foliage to help visitors plan their trip during the peak of the season, when the color is at its best.

There are no eating facilities in the White Mountain section of the Kancamagus Highway. A picnic lunch is recommended, and there are variety stores at either end of the highway which supply picnic and camping needs.

The Kancamagus Highway (Route 112) intersects Interstate 93 from the west, and Route 16 from the east. The highway is clearly marked.

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