I flew back and forth between Denver and San Francisco for an MIT field trip at the end of August 2015, when the western US was filled with smoke. The Utah and Colorado end of my flight is described in my previous post, which trailed off when Utah’s basins were lost from sight.

Here, we pick up again from San Francisco, crossing a beautiful, drought-stricken state. At San Francisco’s latitude, California can be easily divided into four regions running from north to south. The westernmost, and perhaps the most familiar of these, is the Coast Range. This includes the hills of San Francisco, which trap the city’s fog and keep it condensed around the bay in summer. Above the fog, these hills are golden.

California Coast Range

I spent the end of my flight speculating about this apparently bright yellow soil. I found my answer when we drove through the area in the afternoon. It isn’t California’s dirt that’s golden (except in the old mines) – it’s the grass. These hills are pleasantly rounded, grass-covered, and asymmetrically dotted with green shrubs. They form a pretty, arid landscape under the blue Californian sky.

In the northwest or upper-right corner of the photo, the hills flatten out, and California’s Central Valley begins. This is the state’s prime fruit and vegetable basket. Rich soil has poured off the surrounding mountains to fill in a flat basin, and – until recently – the thick soil held good reserves of groundwater. After four years of drought, farmers have pumped a good deal of these reserves. In the past two years, the change has become large enough that GPS devices (used to measure earthquakes) can detect the ground shifting as the water disappears. In the Valley, like most agricultural parts of the US, the property boundaries are square, and the rivers are slow and meandering. Smoke has pooled in the valley today, tinting it blue.

Central Valley farms

Where the valley ends, one of the most spectacular mountain ranges in the US begins. The Sierra Nevada rises from nearly sea level to peaks that tower above 14,000′ (4250m). This range is home to famous landmarks like Lake Tahoe and Yosemite, whose loveliness my field group documented on the ground. ‘Sierra Nevada’ means the snowy range, and in a normal year these pale peaks would be snowcapped even in August. This is not a normal year, and there will be almost no summer snowmelt to top up San Francisco’s reservoirs this autumn.


This photo shows the Sierra’s dramatic eastern edge. The land here falls from peaks above 4000m (13,000′) to sandy plains only 1500m (5000′) high. The drive down this edge of the mountains is steep enough to make your ears pop not once but two or three times. The eastern edge, now hidden in smoke, is California’s forth and perhaps least-famous region, Owen’s Valley, described in 1903 by Mary Austin as “the land of little rain”. This arid land – once famous for its orange crops, and still used for cattle farming – lies in the rain shadow the Sierras. Its lakes are salty, and its slight water resources are now drained by aqueducts pulling water to make a different desert bloom: Los Angeles.