Recently accelerated erosion in the bed of Cache Creek testifies to a big change upstream: a switch in the drainage of Clear Lake. Photos by Andrew Alden

The rainy, dynamic terrain of the northern Coast Range features many rivers, but very few natural lakes. Any basins that may form by tectonic activity have short lives. Either they fill up with sediment, or rivers erode into them and they drain dry.

Clear Lake, by far the largest lake in the Coast Range, has sediments in it dating back some half a million years. The lake's basin never seems to fill with mud, although Clear Lake has always been shallow. Apparently something builds up the basin's sides whenever the lake outlet threatens to cut downward far enough to drain the lake dry. In recent geologic time, Clear Lake has drained eastward and westward at different times as one exit or the other has been plugged. You can visit both exits and glimpse the geologic evidence yourself.

Today Clear Lake drains east to the Central Valley down Cache Creek through a rugged canyon with high, steep walls. But biologists tell us that the fish of Clear Lake come from both the Russian River and the Central Valley. Without that clue, we might not make much of the straight little valley northwest of Clear Lake, just a few meters above the present lake level. Created by slow faulting, the valley holds some bits of water called Blue Lakes. What could have made Clear Lake drain in this direction?

Clear Lake shows signs of having drained northwest, through Blue Lakes, as well as southeast through Cache Creek, its current outlet.

The east side of Clear Lake is full of young volcanoes ranging in size from Mount Konocti down to a dozen little cones a few tens of thousands of years old. With that in mind it's easy to picture a large lava flow, or the emergence of a small cone, blocking Cache Creek and causing the lake to rise until it spilled through the valley of Blue Lakes to the Russian River.

You can drive up state route 20 from the north side of Clear Lake into this valley, and it's easy to picture it holding a river. Then the road hits a large bump, a hill right in the middle of the valley, and that seems to be the end of the line. But geologic mapping has shown that this is an ancient landslide.

The bare-topped hill blocking the valley past Upper Blue Lake is a landslide. The forested land in front is a younger delta fan.

A look at the topography here is instructive, and Google Maps' "terrain" setting is an excellent way to do that. The photo above was taken from the middle "20" symbol and the hill is marked with an asterisk.

Annotations: V symbols are delta-forming drainages, S marks landslides or possible landslides.

Notice how well-organized the stream drainages are leading down to the lakes. Each of the larger streams has built out a delta of sediment. But west of the lakes is an area of poorly organized, hummocky terrain that closely resembles the body of a landslide. It appears to me that the landslide nearest to Blue Lakes is the smallest and possibly the latest of a whole series.

The native tribes around Clear Lake have old stories of just such a landslide here. If we take them literally, that would place the event within the last few thousand years. The next thing to happen in this scenario is that Clear Lake would rise still further until it overcame the natural dam on Cache Creek and resumed its interrupted eastward drainage. In fact, there's no reason this should have happened only once.

But I mentioned visiting both exits of Clear Lake. Cache Creek goes into roadless country but on the other side, right where it meets state route 16, you can pull over at Cache Creek Regional Park and see this beautiful exposure, about 5 meters high, across the river.

From top to bottom its features are plain as day: a nice flat abandoned floodplain, a layer of coarse river rocks and gravel, the rugged profile of a former streambed, and the tilted bedrock of the Great Valley Sequence. In the typical river, the streambed is cut downward so slowly, only in the largest "hundred-year" storm events, that an exposure like this is never seen. But today's newly invigorated Cache Creek has cut through all this in one clean chop in just a few millennia.

The Panoche Hills provide a splendid overlook to the Great Valley on good days. Photos by Andrew Alden

Most people, it's probably fair to say, loathe Interstate 5 between Redding and Bakersfield. If all you want to do is get from point A to point B, I-5 enables that, but the road is so smooth and the route so straight that the typical driver resents the time spent on it, even at 80 mph and faster. Traffic can reach dangerous speeds, eyes get glazed, attention wavers, and tempers grow short.

As for me, I love the chance to enjoy I-5. I make it a full day's trip to get from the Bay Area to Southern California or the Shasta region, and that gives me a long enough leash to get off the freeway at least once during the day. So I'd like to share with you some of my favorite side trips, and they don't include Santa Nella.

One that I've enjoyed several times goes past the Panoche and Tumey Hills, a big patch of wild land along the east side of the Coast Range managed by the Bureau of Land Management. It's a loop of backroad about halfway to Bakersfield, due west of Fresno. Its north end is Little Panoche Road and its south end is Panoche Road. It includes some dirt road and takes a good hour if you don't stop much. Its two notable landmarks are Mercey Hot Springs and the village of Panoche, in well-hidden Panoche Valley. I've never stopped at either place because it's the countryside that attracts me.

Let's have a look at the geologic map of the area, derived from the nice interactive state geologic map:

Significant map units: Ku, upper Cretaceous marine rocks; Ep, Paleocene marine rocks; E, Eocene marine rocks; QP, Pliocene-Pleistocene nonmarine rocks; Q, modern and recent sediment; Qoa, Quaternary river terraces; KJf, Franciscan complex; um, ultramafic rocks

The bulk of the hills consists of late Cretaceous mudstones, laid down in a shallow sea some 80 million years ago when the Sierra Nevada was a great chain of volcanoes like today's Cascade Range. This sea persisted past Cretaceous time into the succeeding Paleocene and Eocene epochs, perhaps 50 or 40 million years ago. There are rare plesiosaur and mosasaur fossils in the older section and lots of fossil wood throughout. But we don't have time to hunt fossils, although collecting for personal purposes is allowed.

Little Panoche Road runs up the valley of Little Panoche Creek, where at least two levels of stream terraces are visible that testify to different episodes of erosion. They in turn are undoubtedly related to the uplift history of these hills.

Looking across Little Panoche Creek to the north flank of the Panoche Hills.

The road turns south and runs through Little Panoche Valley, passing the low-key Mercey Hot Springs resort. The entrance to the BLM's Panoche Hills Recreation Area is here, too. At the head of the valley the road cuts through Glaucophane Ridge, an unexpected exposure of old Franciscan rocks, then goes straight south across the wide and gentle Panoche Valley and ends at Panoche Road. This pocket of farmland is one of Central California's hidden gems. The south side of the valley presents a striking set of terraces. These are mapped as river terraces but might be lake beds; I haven't looked into the literature. In either case they, too, attest to ancient episodes of accelerated erosion.

Buck Peak, in the Griswold Hills, and river terraces of the Panoche Valley.

Panoche Road west eventually gets you to Paicines, south of Hollister. The road east gets you back to I-5, but all of the crooked part is decent dirt. In wet weather, turn back here, or maybe go a little ways on the paved part and daydream about taking New Idria Road south to the remote ghost town of Idria. The eastern end of the valley has good views of the Panoche Hills from the south, but eventually the road crosses the creek without benefit of a bridge and then turns to dirt.

In fine weather, ford the stream and proceed across some splendid and scenic country.

At this point, you are in the Tumey Hills Recreation Area. These hills are lower and drier than the Panoches, and the rocks along the road are quite different. They are young sandstones and conglomerates formed as the Coast Range began to rise, just a few million years ago.

Watch the road as well as the rocks, but don't miss the landscape. All photos in this post were taken in late March.

When you reach I-5, you'll have a choice of eating places to get you ready to roll again. Maybe, like me, you prefer to think of it as the place where the iron dinosaur used to be. It was the creation of the late Carmon Neff of Chowchilla, and I don't know where it went.

You may listen to the "California's Fire Future" Radio report online, as well as find additional links and resources.

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