A Unique Watershed - Large and consisting of 2 distinct parts:
Every watershed is unique in its own way, but the Pajaro has a number of attributes that make it very different.
No other reasonably large North American river drains a watershed that is as complex or as geologically active as the Pajaro. Only in the Himalaya and Alaska are there possibly watersheds of greater than 1000 square miles with an equal level of active watercourse displacement and contemporary changes in drainage area, and those rely in part on glaciers to block and divert the faulted landscapes.
The Pajaro is unique in that geologic activity must be factored in to an understanding of the dynamics of the river - and thus flood hazard evaluation in a populated area. Ongoing geologic deformation renders constructed features like levees and channels very impermanent. Stream gradients and streambed elevations are changing by feet per century from non-anthropogenic causes (cf, 1906 earthquake and loss of navigability of Elkhorn Slough to the commercial steamer carrying Watsonville cargo to Moss Landing, Loma Prieta earthquake, creep on the Calavaras fault).
To understand these aspects better, it is useful to consider the watershed as being composed of two distinct parts - divided essentially by the diagonal formed by the San Andreas Fault as shown in the following sketch. (The watershed as a whole is much larger than shown here. It stretches further south and east off the bottom right-hand corner by a further distance almost equal to that shown on this sketch, but for purposes of this discussion it is sufficient to confine our attention to the northern half depicted here.)
The San Andreas Fault bisects this region diagonally. Bottom-left we have the Lower Watershed as it passes through Santa Cruz and Monterey Counties. This part is on the Pacific Plate. Upper-right we have have the Upper Watershed as it passes through San Benito and Santa Clara Counties, and it sits on the North American Plate.
The Lower Watershed is travelling north with respect to the Upper watershed - effectively stretching the latter along its length.
Lower Watershed, Santa Cruz and Monterey Counties:
The Pajaro Watershed is unusual and any traditional engineering solutions to flood problems will have to accommodate the unique geology and hydrologic character of the basin.
The headwaters of the basin are on the North American tectonic plate, but the primary plate boundary represented by the Calavaras and San Andreas Fault zones separates the mouth of the present river from its historic source areas. Active transform faulting has repeatedly and progressively modified the course of the river that today bears the name Pajaro. The unusual shape of the watershed itself, with a long source area far south of the outlet is the result of continual stretching of the watershed by active faulting that pulls the lower river northwestward, farther and farther from its headwaters.
Much of the lower river, west of the San Andreas Fault Zone, does not flow in a valley of its own making. The original course of Corralitos Creek in Santa Cruz County (seen on the sketch entering the main Pajaro body just upstream of Watsonville from the northwest and generally following the Zayante Fault) and its alluvial aquifer have now been taken over by the Pajaro River system. The ancestral Pajaro River has been repeatedly offset northward by right-lateral fault offset, sometimes emptying to the coast through Elkhorn Slough at Moss Landing, and other times commingling with Corralitos Creek as it does today. California’s State Geologist, Olaf Jenkins (1973) postulated that landslides near Chittenden Gap, forming Lake San Benito and later Lake San Juan that repeatedly spilled and scoured overflow channels in the Carneros Creek/Elkhorn Slough area, might have repeatedly dammed the main river. Even today, during flood stage, the lower river flows to the sea at Moss Landing. Jenkins reasoned that these changes are geologically contemporary, having occurred in the last few thousand to 20,000 years at the most. Fundamental evidence for the very young character of this lake and its overflow is the fact that the lake shorelines are evidently not tilted or deformed, despite being astride two active faults, and finding that the lake sediments contain a fully contemporary local flora and fauna.
It is critical to appreciate that Corralitos Creek and its presumed tributary Aromas Creek did not capture the Pajaro River, but instead a great lake dammed by faulting and/or landslides spilled catastrophically into what we now call the Pajaro Valley. This explains the lack of terraces and floodplain deposits in the lower Pajaro Valley, and the massive Lake San Benito silts that now blanket the lower valley to support its agriculture.
Because the river that now flows through it did not form the lower Pajaro Valley, the watercourse is inherently unstable. Fluvial geomorphology recognizes this condition as “overfit”, with the natural watercourse being too big for its channel. Coupled to this inherent instability is the fact that the lower Pajaro Valley is traversed by the San Andreas Fault and the subsidiary Zayante-Vergeles fault system. These are all among the most active terrestrial fault systems on the North American continent. The 1989 Loma Prieta earthquake apparently deformed the Pajaro River levee system. Today the lowest point in the Pajaro Valley is not the Pajaro River but is a small overflow watercourse along the extreme south side of the lower valley. Based on undercutting of the hillsides at the south edge of the present Pajaro Valley and preserved cutoff meanders there, the southernmost edge of the valley has been the lowest point for at least several hundred to several thousand years.
It is thus perplexing that the present river course and levee system coincide with the lower Corralitos Creek channel. Based on the early maps made shortly after statehood in 1850 and local place names, a grazing wetland commons existed in the Mexican Ranchero period in the area still known as the Vega (historically Rancho Vega del Rio Pajaro). The vega meadows here were apparently flood irrigated regularly to constrain land use and thus provided a grazing Mexican land grant until Statehood and private (Porter) ownership. The Vega is adjacent to a spot on the original river where the river was straightened after the boundary between Santa Cruz and Monterey counties was established (California Historical Survey, 1923). An alluvial thalweg (central river channel) is now buried beneath the levee system and has been the locus of flood outbreaks from at least the 1930s through 1995. All of the positions of today’s levees crossing the 1854 channel position are sites of piping and passage of river water under the levees during high water as seen in 1995 and 1998.
Upper Watershed, San Benito and Santa Clara Counties:
The upper watershed of the Pajaro system is at least as complex as that of the portion west of the San Andreas Fault. There is indirect geologic evidence that Santa Clara Valley from San Jose southward through Morgan Hill and Gilroy may have been the course of a major river carrying coarse gravels southward toward the present Pajaro River and that a lake in San Benito County later spilled northward along Coyote Valley into San Francisco Bay (Iwamura, 1995). An open and porous alluvial gravel characterizes the near surface substrate beneath both the north-flowing Coyote Creek and the south-flowing Llagas and Uvas Creek valleys. A very low gradient watershed divide near Morgan Hill has southward flow in a shallow subsurface aquifer, presumably recharged by Santa Clara Water District facilities from California Water Project sources (Anderson Reservoir) and from locally captured and diverted watercourses. Where this shallow gravel aquifer is exposed in the bank of the Pajaro River, along the westernmost Santa Clara -- San Benito County border, many cubic feet per second of water flow continuously into the Pajaro River. These high water tables were recognized long before the San Luis Project brought Mt. Shasta water into southern Santa Clara and northern San Benito counties. The high groundwater levels are recognized as a particular agricultural problem in San Benito County where some are saline.
The thick uniform silt deposits of Northern San Benito and Southern Santa Clara counties are themselves enigmatic. Jenkins refers to them as “Pleistocene” meaning of Pleistocene age (greater than 10,000 years ago) and draws parallels with glacial age origin silts. Indeed, the surface deposits of lakebed silts are remarkably uniform fine sandy silt similar to glacial origin rock flour in both texture and lack of chemical weathering. But calling upon an ancestral San Joaquin River system to deposit these silts from the Sierra Nevada is, at present, not demonstrated. Jenkins hypothesizes that the silts may be derived locally from the older Purisima Formation (locally now called the Etchegoin Formation east of the San Andreas Fault). Subsurface deposits of northern San Benito County are characterized by localized sands and gravels that appear to be river deposits embedded in silts formed in shallow ephemeral lakes. These are then buried by the more uniform overlying silt lakebeds. It is these surface lake silt unit(s) that have been transported downstream to blanket the lower Pajaro River Valley. It is not clear that they are being eroded from agricultural fields upstream, and may simply be carried in flood flows from upstream bank erosion.
The Calavaras, San Andreas, and Sargent fault zones define much of the course of the present tributaries of the upper Pajaro River system. These right-lateral strike-slip plate-bounding fault systems essentially lengthen the headwaters of the Pajaro River, repeatedly moving the upper river system southward 10’s of kilometers relative to the Pacific Plate. The Old San Juan Stage Road between Salinas and San Juan Bautista appears to follow an abandoned course of what is now called the San Benito River after that river was pulled northward on the west side of the faults to join the upper Pajaro River. All of this may have happened during as little as a few hundred or thousand year period of lakes being dammed and spilling before the river ultimately broke through the Chittenden water gap to spill westward rather than southward. It is interesting to note that this rare example of a true water gap in western United States is actually called Chittenden Pass. A water gap is a pass through a mountain range or ridge cut by water. These are generally found in places like the Appalachians where a very old river is able to keep flowing while mountains are arched upward beneath it or while erosion lowers the river across a buried bedrock feature. Chittenden Pass is indeed a narrow part of the new river valley but cut by catastrophically spilling water.