Another Freshet

December 31, 2005
Sonoma County, California

Another New Year's Eve Freshet

Floodplain constrictions

During major storms, the floodwaters of the Laguna cover corn fields, pastures, and vineyards in a wide, slow moving arc which moves north towards the Russian River. But the floodplain does not hold water at even heights, as shown in the preceding table. Constrictions, both natural and man-made, retard the leveling process even as the floodplain continues to fill from the south and drain to the north. These constrictions create an uneven sea of subtly different iso-heights.

Just west of Mirabel Road, the combined waters of Mark West Creek and the Russian River meet a geologic constriction—the Forestville Narrows—that inhibits the water’s forward motion and reduces its speed.  Over millenia, sediment dropping out of the water at this point has raised the surface elevation enough to establish a natural ponding effect to the east of the canyon’s entrance. During summer flows this creates the still waters enjoyed by the area’s residents and recreational visitors.

When large volumes of water arrive at this point during winter storms, this sediment bar—and the Forestville Narrows itself—effectively create enough of a constriction to cause the normal flow of the Russian River (from Healdsburg towards Guerneville) to be shunted up the Mark West Creek floodplain towards Trenton-Healdsburg Road.  When this backwater effect is occurring, the floodplain of Mark West Creek can become inundated to a great depth, over-topping the grapevines, and giving the area an appearance of a vast sea.  On December 31, 2005, the depth of the water on the floodplain near Wohler Road was 16 feet in some places, while the maximum depth of the water in the channel itself, at the Wohler bridge, was 36 feet!

Here are the other major constrictions within the floodplain:

1. The first constriction upstream of the Russian River, is Trenton Bend, located on Mark West Creek 0.51 miles downstream from Trenton-Healdsburg road, near the confluence of Castens Creek.  Here the creek bends sharply north to pass between the shoulders of two small hills.  The width of the floodplain narrows to 442’.
2. Ritchurst Knob is the last major vestige of the Coast Range and, conversely, the first solid bedrock constriction west of the Santa Rosa Plain. Mark West Creek follows a wide arc as it rounds the knob, and at point 27.46, just upstream of the Windsor Creek confluence, the floodplain narrows to 697’.
3. Lomita, the Eucalyptus hill located a quarter mile north of River Road, provides a tight narrows for Mark West Creek. When the stage remains below 57’ the floodplain is only 138’ wide, but when the stage exceeds 57’ the floodplain extends around the eastern side of the hill, turning it into an island, while effectively removing the constriction.
4. The River Road causeway extends 1537’ across the 70’–elevation floodplain, blocking three-fourths of the natural flow line of the Laguna—Mark West complex and restricting it to the 500’ long bridge. This bridge has an elevation of 84’, a virtually flood-proof height.
5. Ballard Hill, on Denner Ranches, at one time formed the northern boundary of Ballard Lake.  The Laguna at this point was dredged, decades ago, to remove the lake; nevertheless, natural aggradation at this point has recently occurred causing the reappearance of the former lake. In addition, Mark West Creek has been altered dramatically at this point, drawing water down from its natural course above River Road.  This new creek channel sends a substantial amount of additional water and sediment around the eastern side of Ballard Hill, to join the Laguna behind the natural constriction.
6. The Guerneville Road causeway extends 2165’ across the 71’ floodplain, blocking four-fifths of the natural flow line of the Laguna. Situated just downstream from the Santa Rosa Creek confluence, it blocks the flow of the combined Laguna/Santa Rosa subwatersheds. This ranks as one of the major constrictions of the floodplain.  A higher elevation of flood water was detectable on December 31, 2005, even with the simple observation and calibration methods used. The bridge has an elevation of 80’, making it unlikely than any flood event would overtop it.
7. Delta Pond is constructed on the former delta of Santa Rosa Creek, which in former times had a braided network of secondary channels in this area. This was the final floodplain of Santa Rosa Creek, and was, from all evidence, the likely spot where fine sediments reached their dropping point. Today Delta Pond occupies 90 acres of this floodplain, while its confining levees occupy another 10 acres of the floodplain. This displacement causes the Laguna’s water, waiting to move through the Guerneville Road constriction, to back up and overflow the land to the south. Aggradation in this area is expected to be higher than elsewhere.
8. The Occidental Bridge causeway extends 3061’ across the 73’ floodplain, blocking 90% of the natural flow line of the Laguna and restricting it to the 250’ long bridge. This bridge has an elevation of 71’, while the approaches have an elevation of 69’.  This bridge was over-topped on December 31, 2005.
9. The Highway 12 Bridge causeway extends 2717’ across the 72’ floodplain, blocking 90% of the natural flow line of the Laguna and restricting it to the 250’ long bridge. This bridge has an elevation of 72’. This bridge was over-topped on December 31, 2005.
   To the south of Sebastopol the numerous oxbows, summertime ponds, and braided channels of the Laguna overflow to a lesser degree.  The natural high spot near Cooper Road was historically the fording spot for the Laguna allowing wintertime travelers their best chance to cross the floodplain between Sebastopol and Llano.
10. Near the end of Morse Rd the Laguna is constricted by natural topography to the east and west.  This constriction is about 1500’ northwest of the Todd Road bridge. The depth of the floodplain drops most dramatically in this reach—testament to the significance of this constriction—losing five feet of elevation in three-fourths of a mile, immediately after passing through the narrow point. The width of the 78’ floodplain at the Morse–Todd constriction is 1200’.
    Further south, the waters of Blucher Creek—the watershed’s only major tributary situated to the west—join the Laguna near present day Todd Road. Local residents know this area for its flood prone nature: Todd Road is closed frequently each winter due to flooding.  The Todd Road bridge—at elevation 80’ and with a width of 150’—is not a significant constriction by itself. Most of this flooding is the natural effect of Blucher Creek reaching its terminal confluence and splaying out onto the Laguna’s floodplain.  Agriculture in the area fares well with these inundations, but local residences along the Todd Road lowlands are repeatedly subject to property damage. Nevertheless, residents on Powers Road experienced flooding for the first time on December 31, 2005.
11. At Llano Road, the Meadowlane Ponds—used to store clean water from the adjacent Laguna Wastewater Treatment Plant—have been constructed on the site of a former lowlands.  The situation of these ponds creates a major constriction: in major floods the reconstructed Laguna (just to the west of the ponds) is unable to handle the volume of water pushing through the constriction. The 82’ floodplain is just 268’ wide, giving this constriction first prize, other than the bridges, as the narrowest part of the Laguna. The four foot drop in floodplain elevation immediately after this constriction demonstrates its significance in retarding forward motion.
    The Llano Road bridge, so close to the Meadowlane Ponds, is not a significant constriction in itself. Its top is at 84’; its width is 170’. This bridge was over-topped on December 31, 2005 with waters reaching 85’ east of the bridge.
12. The Stony Point bridge causeway extends 1860’ across the 89’ floodplain, blocking 90% of the natural flow line of the Laguna and restricting it to the 235’ long bridge. This bridge has an elevation of 87’.  This bridge was over-topped on December 31, 2005.

December 31, 2005

• Floodplain extent
• Floodplain waterways
• Waterway contributions
• Another New Year's Eve Freshet
• Newsworthy
• A local story
• Reconnaissance
• Calibration
• Observations on the data
• Laguna-Mark West
• Floodplain determination
• Floodplain elevation
• Floodplain constrictions
• Road closures
• Property damage
• Historic comparison

Text, photos and maps copyright © 2008 Joe Honton