Upper Watershed Delivers Water and Sediment
Draining a total watershed area of 37 square miles before leaving the hills, San Francisquito Creek is the largest stream on the western margin of San Francisco Bay. This map depicts two parts of the historical creek system, the upper watershed, which collects and delivers water and sediment (solid black boundary), and the alluvial fan, which receives that water and sediment (dashed black boundary). Together they form an hourglass shape, joined together at the waist where the creek exits the hills (point A, near the intersection of Alpine Road and Junipero Serra Boulevard). The watershed can be further divided into subwatersheds, including the Bear Creek, Corte Madera Creek, Los Trancos Creek, and San Francisquito Creek subwatersheds.

During winter storms, the upper watershed funnels water and sediment into San Francisquito Creek. Starting as rain falling on the hillsides, the water flows downhill, then downstream, picking up sediment. The muddy water courses down tributary streams, often eroding bed and bank, until emptying into San Francisquito Creek and rushing toward the flatlands.

The Creek Builds the Alluvial Fan
During floods, the sediment-laden waters of San Francisquito Creek spread out and deposit the sand, silt, and gravel carried from the hills in lobes and layers over the flatlands. Over tens of thousands of years, the creek changed course repeatedly, building up a fan-shaped deposit of sediment that radiates out from the mouth of the canyon near point A. This resulting alluvial fan is the landform upon which much of Palo Alto, East Palo Alto, Menlo Park, Atherton, and the Stanford campus now sit.

The fan now comprises thick deposits of sand and gravel. Creek water easily permeates these porous sediments; San Francisquito Creek loses significant flow into the alluvial deposits. Many smaller neighboring creeks flowing on to alluvial fans historically never reached the bay, but spread out on the flatlands and sank in. Water lost this way ultimately reaches the bay as groundwater.

Starting a few thousand years ago, tectonic uplift of the Stanford area caused San Francisquito Creek to gradually abandon the upper portion of its fan and deposit its sediments farther and farther downstream. Incision of the creek channel into the older sediments near the upstream end of the fan confined the creek to a deep channel. Downstream, however, the creek continued to build its alluvial fan. The portion of the fan that East Palo Alto now occupies dates from a time when the creek splayed out near point B.

At present San Francisquito Creek is limited from much further fan building. Incision and flood control projects keep the creek from overflowing its banks in all but major floods. In addition, Searsville Lake traps the sediment from the Corte Madera subwatershed, significantly decreasing the material available for fan building. The creek now deposits relatively small amounts of sediment within the channel downstream of point B, and is building a small delta into the bay at the mouth of the 1930s-era engineered channel.

Natural Levees Give Shape to the Fan
When a creek overflows its banks and floods the flatlands, it drops a large portion of its sediment immediately adjacent to the channel where the current begins to slow. Over time, a low ridge, or natural levee, builds up along the creek’s banks. Today a prominent natural levee, easily seen in the topographic contours on the inset figure, borders San Francisquito Creek. Unlike the hills, where creeks occupy the bottoms of valleys, creeks in the flatlands flow down the center of low ridges of their own creation.

Clues to Where the Creek Once Flowed
Former courses of the creek generally followed the centerline, or crest, of old natural levees that are still visible in the topography on the alluvial fan. The inset figure is a digital shaded-relief map exaggerated to make these subtle natural levees visible. Dashed blue lines along the centerline of the levees approximate the former stream courses, now erased by time. The larger map shows all of the former stream courses found in this manner.

These former stream courses and their deposits range in age from very recent to a few tens of thousands of years old, with the most recent deposits found downslope of point B. Historical maps of East Palo Alto show an abandoned stream channel still visible along the crest of the northernmost levee. Point C near historic Ravenswood is the protrusion of that levee into the tidal marsh.

The subtle levee topography on the San Francisquito alluvial fan has influenced the distribution of vegetation types and patterns of flooding. Historical maps show willows and wetlands occupying the low areas between levees, especially near the bay shore, and live oak and laurel occupying the levees. In the 1998 flood, the largest of record for San Francisquito Creek, water spilled out of the channel and flooded the low-lying areas between the levees, leaving the levee areas dry.

Alluvial Fans in an Urban Setting
Recognizing how alluvial fans form and how we have modified them can help us understand some of the hazards encountered with urban streams. Flooding is a natural occurrence that became an urban hazard. Historically floodwaters overtopped the creek banks and spread over the flatlands on a regular basis. Today, homes and businesses cover the flatlands. Making the problem worse, pavement and rooftops prevent infiltration of rainwater, thus increasing the frequency of flooding relative to natural conditions.

The natural processes of erosion and sedimentation are also problems in an urban setting. Sediment can choke the lower portions of stream channels on alluvial fans, diminishing their flood capacity. Dredging can deepen the channel to prevent flooding. Alternatively, levees can be built or raised along the creek bank. Upstream reaches erode naturally, and measures are often taken to protect creekside property, such as reinforcing the banks or moving structures away from the creek.

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San Francisquito Watershed Information

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