Theme for 1996:
A Prehistoric Look at California Rainfall and Floods

Data on California's Extreme Rainfall from 1862-1995

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Abstract:

This is an account of some of the measured storms within the memories of recent California inhabitants. Some of these storms have remarkably large return periods (Map 1). They are not limited to the windward slopes like winter time orographic storms, (Map 2) they occur randomly distributed geographically as well as by season.

This is an updated version of the paper offered here two years ago. The data base has been expanded and there have been two of California's most costly storms in terms of 1996 dollars, have occurred since the last report. The main deference beside the expanded data base is the up date to include the great storms of 1995. The storm maps showing lines of equal return period which were included in the proceedings of this conference two years ago are not repeated Maps 4 and 5 for the storms of 1995 are included in this study.

Paleoclimatic Evolution of Santa Barbara Basin During the Last 20 k.y.: Marine Evidence from Hole 893A

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Abstract:

The paleoclimatic record for the last 20 k.y. has been examined within a detailed radiocarbon age framework for Hole 893A, Santa Barbara Basin, California, using benthic and planktonic foraminiferal oxygen isotopes and assemblage changes in planktonic foraminifers. The last deglaciation is recorded as two steps (Terminations IA and IB) separated by a warm pause, correlative with the BØlling/AllerØd lnterstadial (15.2 to 13 ka), followed by Younger Dryas cooling (13 to 11.1 ka). This, in turn, was followed by the warmth of the Holocene. Changes in the planktonic foraminiferal assemblage suggest that average sea-surface temperatures in the basin increased from ~7°C during the last glacial maximum to ~15°C during the late Holocene, an estimated change larger than shown in earlier work.

Hole 893A consists of alternating laminated and nonlaminated (massive) sediment intervals. Laminated intervals were deposited at times of low dissolved oxygen levels in the basin that precluded support of an active benthos. Massive intervals resulted from bioturbation associated with more oxygenated waters. The basin was relatively well ventilated during the glacial to near-glacial intervals (last glacial maximum and Younger Dryas cooling) and poorly ventilated during warm intervals (BØlling/AllerØd lnterstadial and Holocene during the last 11 k.y.). Thus, the sequence records oscillations between two fundamentally different states, one marked by bottom waters with low oxygen content associated with warm intervals, the other with relatively oxygenated waters associated with cold intervals.

We interpret these oscillations in sediment facies to reflect changes in the source of intermediate waters, with a greater proportion of oxygen-rich waters originating from a more proximal source during cooler intervals compared with a distal source of oxygen-poor waters during warmer intervals, including the present day. The stratigraphic records examined here strongly indicate that, as in the North Atlantic, fundamental changes occurred in North Pacific intermediate water circulation during the latest Quaternary. These were closely synchronized with global climate change and with paleoceanographic events in the North Atlantic. It is possible that changing strength of oceanic conveyor circulation, currently transporting waters from the North Atlantic to the North Pacific, may have in part caused changes in oxygen content of upper intermediate waters near the coast of North America, including the Santa Barbara Basin. Alternatively, the inter-ocean paleoceanographic changes were linked directly through global climate change transmitted through the atmosphere rather than through changes in the strength of the oceanic conveyor. In this case, severe cooling during the last glacial maximum and the Younger Dryas episode led to the production of intermediate waters at high latitudes in the Pacific Ocean that influenced Santa Barbara Basin ventilation. Of proximal origin, these were young, well oxygenated waters. A third, more likely, hypothesis is that both factors in combination played a role in influencing the ventilation history of the basin. This investigation indicates the existence of tight coupling between changes in the atmosphere-ocean-cryosphere during the latest Quaternary.

[Proceedings of the Ocean Drilling Program, Scientific Results, 1995, vol. 146 (Pt. 2), Kennett, J.P., Baldauf, J.G., and Lyle, M. (Editors)]

[Summarized at the Symposium by Charles Dailey and reprinted in these proceedings with permission.]

A 2000 yr Record of Sacramento-San Joaquin River Inflow to San Francisco Bay Estuary, California

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Abstract:

Oxygen and carbon isotopic measurements of fossil bivalves (Macoma nasuta) contained in estuarine sediment are used to reconstruct a late Holocene record of salinity and stream flow in San Francisco Bay. Discharge into the bay is a particularly good indicator of paleoclimate in California because the bay's influent streams drain 40% of the state. The isotopic record suggests that between about 1670 and 1900 calendar years (yr cal) B.P. inflow to the bay was substantially greater than the estimated prediversion inflow of 1100 m³/s. An unconformity representing a 900 yr hiatus is present in the core between 1670 and 750 yr cal B.P., possibly caused by a major hydrologic event. Over the past 750 yr, stream flow to San Franciso Bay has varied with a period of 200 yr; alternate wet and dry (drought) intervals typically have lasted 40 to 160 yr.

[Geology, April 1996, vol. 24, no. 4, p. 331-33]

[Summarized at the Symposium by Charles Dailey and reprinted in these proceedings with permission.]

Extreme and Persistent Drought in California and Patagonia During Mediaeval Time

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Abstract:

Studies from sites around the world have provided evidence for anomalous climate conditions persisting for several hundred years before about AD 1300. Early workers emphasized the temperature increase that marked this period in the British Isles, coining the terms 'Mediaeval Warm Epoch' and 'Little Climatic Optimum', but many sites seem to have experienced equally important hydrological changes. Here I present a study of relict tree stumps rooted in present-day lakes, marshes and streams, which suggests that California's Sierra Nevada experienced extremely severe drought conditions for more than two centuries before AD ~1112 and for more than 140 years before AD ~1350. During these periods, runoff from the Sierra was significantly lower than during any of the persistent droughts that have occurred in the region over the past 140 years. I also present similar evidence from Patagonia of drought conditions coinciding with at least the first of these dry periods in California. I suggest that the droughts may have been caused by reorientation of the mid-latitude storm tracks, owing to a general contraction of the circumpolar vortices and/or a change in the position of the vortex waves. If this reorientation was caused by mediaeval warming, future natural or anthropogenically induced warming may cause a recurrence of the extreme drought conditions.

[Nature, 16 June 1994, vol. 369, p. 546-549]

[Summarized at the Symposium by Charles Dailey and reprinted in these proceedings with permission.]

Paleohydrologic Bounds and the Frequency of Extreme Floods on the Santa Ynez River, California

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Abstract:

Many rivers in the western U.S. are flanked by a stair-step series of terrace surfaces. These terrace surfaces are abandoned flood plains that range in age from several hundred to tens of thousands of years. The soils and stratigraphy under these surfaces record the time interval since the last major flood inundation. The deposits under these surfaces are stream-transported flood plain sediment that is highly-erodible, and thereby reliably records substantial inundation. Preserved, non-inundated surfaces of known age form conservative limits for the paleostage of past large floods. These paleostage limits can be input into a step-backwater model to estimate the maximum discharge that would not significantly inundate, and therefore significantly modify, a particular geomorphic surface. This maximum discharge, together with the age of the surface, forms a conservative limiting bound on peak discharge over a long time period. These bounds are not actual floods, but instead they are limits on flood magnitude over a measured time interval. In this way, these bounds represent stages and discharges that have not been exceeded since the geomorphic surface stabilized.

Following the framework introduced by Stedinger and Cohn (1986), this type of flood record spanning hundreds to thousands of years can be input into flood-frequency calculations. These long-term paleohydrologic bounds accurately portray the ability of a specific basin or region to produce extreme floods and significantly narrow the confidence intervals around predicted flood magnitudes at long return periods. Including paleohydrologic bounds in the flood frequency calculations indicates that the flood fiequency curve has a fundamentally different trend at long return periods. That is, extrapolating only from the record of annual peak discharge estimates leads to return periods for large floods that are orders of magnitude shorter than if the paleohydrologic bounds are included. In the case of the Santa Ynez River at Bradbury Dam, this means that a discharge of spillway capacity (160,000 cfs) has a calculated return period of more than 6,000,000 years when paleohydrologic bounds are included in the frequency analysis, as compared with a calculated return period of less than 2000 years when flood frequency is calculated based on the record of annual peak discharge estimates.

For dam safety, the critical issue is not the accurate estimation of a complete record of floods well within the operating range of the structure, but rather the frequency of floods that could challenge the operational capacity of the structure. The key issues are the precision of the frequency estimate of such large floods, and the probability that the operational capacity of the dam will not be exceeded. Floods near the magnitude of the paleohydrologic bounds are direct indicators of the likelihood of large floods that might compromise dam safety. The results of paleoflood studies in California, Oregon, and Utah demonstrate that discharges with calculated annual probabilities of 1 in 10,000 are in the range of five to 20 percent of the hypothetical Probable Maximum Flood (PMF).

A Thousand Year Flood Record from Little Packer Lake, Glenn County, California

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Abstract:

… An alternative approach is to reconstruct past flood events by geomorphological or biological evidence, such as the existence of alluvial deposits above the normal channel of a river, or the presence of flood induced scars on river bank trees. This kind of evidence can offer a long-term perspective on flood frequencies in particular watersheds, but in most cases such records are incomplete. A problem is that major floods are inherently destructive; they tend to remove evidence of previous floods. In this paper we draw attention to a new source of paleoflood data: the sediments of oxbow lakes.

Tree-Ring Reconstructions of California Precipitation Variability

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Abstract:

Tree growth indices from moisture-sensitive sites throughout California provide the most accurate, detailed information on precipitation variability over the last 400-500 years. Tree-ring reconstructions are not exact records of past climate, however, so the validity of patterns identified in reconstructions needs to be evaluated relative to quantitative reconstruction error estimates.

In this study reconstructions of Southern California regional average precipitation and Sacramento Basin riverflow are analyzed to identify variations in the frequency of extreme events over the last four centuries. The reconstructions are also compared with each other to determine the extent to which extreme events have occurred simultaneously in the two regions. The statistical validity of all results is assessed using Monte Carlo simulations to account for reconstruction errors. The results that there have not been major changes in California's precipitation regimes during the last four centuries. While much of the more subtle variability is masked by reconstruction uncertainties, it does appear that the 20th Century climate has been characterized by fewer extreme dry years and more wet years than the preceding three centuries, particularly in Southern California.