Another New Year's Eve Freshet

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Floodplain extent

The floodplain of the Laguna de Santa Rosa is one of the watershed’s most recognizable features. It is often seen from the Occidental Road bridge inundated with water: a freshly emerged lake, cocoa colored, calm, going nowhere.  Only the birds break the still surface of its waters. And more frequently than not, the vista from the bridge on these high-water days is sunny, providing viewers with a reflecting light that accentuates the thin line of trees standing submerged along its summer season embankment.

This vista has an intrinsic character that fills me with awe each time I see it anew. But before it is dismissed as the musings of a poet, consider the natural cycles of weather and water that inexorably lead up to it. Three natural cycles provide the energy flux that leads, in predictable fashion, to this state: wintertime Pacific storms, upper tributary runoff, and the low gradient course of the Laguna.

Part of the Laguna de Santa Rosa floodplain; shown here is the area south of River Road, on January 6, 2006During the December to March rainy season, moisture from the Pacific Ocean is drawn from its subtropical high pressure zone into California’s wintertime low pressure cells in staccato periods of three- to seven-days.  Smaller storms last six to twelve hours, while major storms can have a duration of up to 30 hours. The periods in between storms are typically cloud free and unaccompanied by winds.  This periodicity, of rain and shine, sets the stage for the appearance of the still, sunny days that accompany the floodplain’s reappearance.

Natural runoff from the watershed’s tributaries in the Taylor Ridge area are joined by a similar flow arriving from the Gold Ridge Hills—via Gossage and Blucher creeks—to form a flood wave crest within hours of a storm’s start.  This crest rises in height and increases in volume as the storm progresses, and falls almost immediately after it has finished: this is the so-called “flashy water” effect.

The low gradient course of the Laguna dampens the speed of the water from these tributaries, retarding its arrival at the Occidental bridge by twelve to fifteen hours. By that time, the Santa Rosa–Matanzas Creek watershed has gone through a similar process, but slightly in advance, and the Laguna’s height at the Delta Pond confluence is higher than its upstream course at Occidental Rd throughout the storm and immediately afterwards for a few hours. The arrival of the flood wave crest from the southern tributaries after the rise in Santa Rosa Creek’s water at its juncture (two and a quarter miles miles downstream), halts the forward movement of the water, pushing it laterally outward, onto the floodplain.

Thus the stage is set and the actors are in motion. With the precipitation complete, the winds calm, and the arriving water having nowhere to go, the Laguna swells onto the floodplain, its sediment-laden waters reflecting its peculiar hue: the Laguna reappears.

In larger storms, the dynamics are grander and the floodplain extends north of Guerneville Road. During those events, the former Lake Ballard reemerges—among the stubble of corn stalks—in the low lying areas to the east of the watercourse. These storms trigger out-of-bank conditions at several other well known spots as well: the Balletto–Dei lowlands; the southwestern corner of Sebastopol’s Meadowlark Field; lower Brown Farm; the Blucher Creek confluence at Todd Road; and the hay field east of the Bellevue–Wilfred channel.

During major events, the floodplain extends all the way to the Russian River, covering vineyards along the edge of Mark West Creek, and closing the Wohler Road bridge.

But it is only a rare event, like the one that occurred on December 31, 2005, that causes these regional flood-prone areas to become connected into a contiguous seventeen-mile long floodplain. On that day, the water’s surface area spanned 7062 acres; an area equivalent to 11 square miles.