The Dimond Canyon water gap

In a city full of geologic features, Dimond Canyon stands out as a classic example of a water gap. But it can be hard to see, even from the prime viewpoint of Leimert Bridge.

Let’s abstract ourselves by studying the overhead views shown in maps. Google Maps, with the terrain view turned on, is where I like to start.

Compare Dimond Canyon, cutting straight through the bedrock ridge of the Piedmont block, with Indian Gulch (Trestle Glen) on the left, a conventional stream-cut valley that fans out against the ridge.

For a starker view of the topography, I like to consult old USGS maps like the 1897 Concord quadrangle, made before most of the area was built up and dug into.

Here we can see that the ridge reaches the same elevation on either side of the canyon — without the canyon cutting through it, this would be a continuous crestline.

There is no sign, either, that Dimond (“Diamond”) Creek cut its way through by headward erosion. That would have left tributaries fingering off on either side, like those visible in the contours above Indian Gulch. Indeed, the single little tributary in the canyon is actually a hanging valley that has to descend steeply as it meets Dimond Creek — not as spectacular as those in Yosemite Valley, but with the same basic configuration.

Finally, we can look at the bedrock evidence in the geologic map.

The whole area around the canyon is mapped as Franciscan sandstone (Kfn), with no hint of faulting or other structure that might have favored the formation of a canyon here. Consider the well-developed valleys above the canyon, guided into existence by the rock-crushing Hayward fault, or the more subtle topographic features where the southern edge of the Franciscan bedrock meets old alluvium.

What we have here, then, is a genuine water gap — a deep pass in a mountain ridge with a stream flowing through it. Geology textbooks will tell you there are two ways to make one. One is for a river to uncover an ancient ridge as it strips the countryside of its sediment cover. The classic case is the Delaware Water Gap in Pennsylvania. The other is for a river to sit there, doing its thing, as the land rises up around it. The Central Valley has good examples at the foot of Del Puerto Canyon . . .

. . . and the Berryessa water gap west of Winters.

What’s odd about the Dimond Canyon water gap is that it’s being carried along the Hayward fault. Every few hundred thousand years, then, it gets itself a new headwater catchment. Today its catchment is Shephard and Palo Seco Creeks. Once upon a time, though, it must have carried the waters of San Leandro Creek. Coming up: Temescal Creek.

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