Patrick Crain, a researcher with the UC Davis Center for Watershed Sciences, studying fish use of the floodplain in the Cosumnes River Preserve. These flooded areas are highly productive and important rearing habitat for Chinook salmon, Sacramento splittail and other natives fishes. Photo by Peter Moyle

By Jeffrey Mount

In December of 2010 we had a remarkable set of storms.  Relentless rain and snowfall hit both southern and northern California.  The news reports about these events followed a predictable pattern, including the inevitable articles that bemoan floodwaters as “wasted” because they discharge to the sea.

This generalization about floodwaters or any fresh water that makes it to the ocean has been around since John Wesley Powell recommended irrigation districts to manage water in the West to prevent “one drop from being wasted to the sea.”  It is a hopelessly out of date concept, however.

First, the hallmark of our Mediterranean climate is alternating periods of over-abundance and scarcity.  The magnitude of this overabundance overwhelms our economic capacity to capture it.

Simply put, it makes no economic sense to build the massive structures necessary to capture and consume the largest floods.  They won’t store water often enough to make them economically viable, never mind the environmental damage caused by such structures.

Flood water flows across a Cosumnes River floodplain and into the Delta in March 2006. The water, carrying plankton, organic material and native fish strengthened the food webs of the Delta and San Francisco Bay. Photo by Jeffrey Mount

Second, floods provide broad beneficial uses that cannot be replicated any other way.  These benefits include water quality, groundwater recharge and habitat.

Winter floods play a key role in improving water quality.  Dams and diversions reduce the amount of water that flows in rivers and into estuaries.  This reduced volume amplifies the negative effects of farm and urban runoff, along with wastewater treatment plant discharges.  Winter flushing flows are integral to and necessary for maintaining better water quality, particularly in the Sacramento-San Joaquin Delta.  The alternative—discharging only clean water—might be a good idea in principle, but, like massive flood control dams, is prohibitively expensive.

An under-appreciated way we recapture floodwaters that flow out of our dams is through groundwater recharge.  During floods, a small, but significant proportion of the water enters the groundwater through floodplain soils and the river channel.  This is especially important on the San Joaquin River and the Cosumnes River of the Central Valley. And we do not take advantage of this relatively cheap source of water storage enough.

The most significant benefit of floods comes from their role in improving habitat in floodplains, rivers, and estuaries.  At the outset, it is important to understand that floods are only natural disasters because humans get in the way.  Floods are an essential ecosystem attribute.

The river and estuarine plant and animal communities of California are all flood-adapted.  Flood flows provide the energy needed to erode, transport and deposit sediment and coarse organic material: a process critical to maintaining healthy channels, floodplains and marshes.  Trees and shrubs along the rivers depend upon floods to establish ground for new vegetation, augment soil fertility, fill wetlands and recharge shallow groundwater.

Flooded forests of the Cosumnes Preserve. Prior to reclamation of Central Valley floodplains, of the Central Valley, vast forests of oaks, cottonwoods, willows and other trees contributed organic material that support abundant insect life in the rivers.

Various native fishes, such as Chinook salmon and Sacramento splittail, take advantage of floods to forage or spawn on highly-productive flooded lands. Other fish, such as the delta smelt and sturgeon, time their movements to take advantage of conditions created by floods.

The great annual migrations of waterfowl along the Pacific Flyway use the wetlands created by seasonal floods. And finally, these floods, when moved slowly across floodplains, help feed the ecosystems of the Delta by delivering abundant plankton and other forms of carbon necessary for Delta productivity.  The loss of this important subsidy of food is considered one of the key stressors affecting the Delta.

Comparison of juvenile Chinook salmon reared on a restored Cosumnes River floodplain (right) and in the river’s main channel. Photo by Jeff Opperman, 2006

The notion of water “wasted” during a flood is outdated and wrong.  These floods provide broad benefits that are typically under-appreciated and rarely figured into cost and benefit considerations in flood management.

However, there are exceptions to this generalization, particularly in urban settings such as coastal southern California and the Bay Area.  Here, antiquated storm runoff systems route floods to the sea through straight, concrete-lined channels that offer none of the benefits cited above.  Worse yet, this runoff is often laden with nutrients, pesticides, viruses and bacteria, leading to beach closures or costly water treatment structures.  Bad urban water management, in this case, genuinely represents water wasted to the sea.

If you haven’t seen or read it yet, the new book, “Managing California Water: From Conflict to Reconciliation” addresses this and many other issues concerning water management in the state. Available in a free pdf version or paperback and e-book format at Amazon.

On Wednesday, March 2, the book will be the subject of a Washington, D.C., seminar that can be viewed online. Lund, Hanak and others will discuss new approaches to balancing economic and environmental goals for water management. The webcast begins at 9:45 a.m. Pacific time. Information: http://www.rff.org/Events/Pages/Water-Conflicts-and-Resolution-Economy-vs-Environment.aspx.

Jeffrey Mount is a geologist and the founding director of the UC Davis Center for Watershed Sciences

Further reading

Ahearn, D.S., J.H. Viers, J.F. Mount, and R.A. Dahlgren (2006), “Priming the productivity pump: flood pulse driven trends in suspended algal biomass distribution across a restored floodplain,” Freshwater Biology 51, pp 1417-1433.

Feyrer, F., T. Sommer, and W. Harrell (2006), “Importance of flood dynamics versus intrinsic physical habitat in structuring fish communities: Evidence from two adjacent engineered floodplains on the Sacramento River, California,” North American Journal of Fisheries Management 26, pp. 408-417.

Fleckenstein, J., M. Anderson, G. Fogg, and J. Mount (2004), “Managing Surface Water-Groundwater to Restore Fall Flows in the Cosumnes River,” Journal of Water Resources Planning and Management 130, pp. 301-31.

Lehman, P.W., T. Sommer, and L. Rivard, (2008), “The influence of floodplain habitat on the quantity and quality of riverine phytoplankton carbon produced during the flood season in San Francisco Estuary,” Aquatic Ecology 42, pp. 363-378.

Moyle, P.B., P.K. Crain, and K. Whitener (2007), “Patterns in the use of a restored California floodplain by native and alien fishes,” San Francisco Estuary and Watershed Science 5(3): 1-27.

Naiman, R.J., H. Décamps, and M.E. McClain (2005),  Riparia: Ecology, Conservation, and Management of Streamside Communities, Elsevier Academic Press: Amsterdam, 430 p.

Schemel L.E., T.R. Sommer, A.B. Muller-Solger, and W.C. Harrell (2004), “Hydrologic variability, water chemistry, and phytoplankton biomass in a large floodplain of the Sacramento River, CA, U.S.A.,” Hydrobiologia 512, pp 129-139.