by Peter Moyle, Jay Lund, Andrew L. Rypel, Carson Jeffres and Nicholas Pinter
Floods and their consequences are a reality for many worldwide, including those living in California. This reality is evidenced by pictures of people stranded on roofs surrounded by water, people paddling down water-filled streets in makeshift boats, and farm fields and orchards covered in standing water. However, there is also growing acceptance that floods are natural, recurring events that have positive aspects, especially where they support migratory waterfowl, enhance fisheries, and sustain wetlands and their high diversity of organisms (Mount et al 2023). In fact, most communities on large rivers globally rely on annual flooding for soil nutrient replenishment, transportation, and maintenance of larger ecosystems that supply food resources (e.g., Amazon, Nile, Okavango, Ganges, Mekong rivers). In California much of our most productive farmland is actually former floodplain, including the once vast wetlands of the Central Valley and the bed of Lake Tulare (Moyle 2023). Most of our cities also are partially built on former floodplains. It is no wonder then that levee building, from the beginning, has been a major activity in the state (Kelley 1989). But the water keeps challenging our defenses, a situation worsening with climate change. This creates major hardships for people and the remaining native biota of California, especially fishes. It is no coincidence that many threatened native species depend on floodplains and their connected wetland and riparian systems. Yet decades of research projects at the UC Davis’ Center for Watershed Sciences on floodplain ecology in California shows that floodplain habitat restoration can benefit fish and wildlife while concurrently reducing flood impacts on the built environment (Opperman et al. 2017).
Understanding large floods and applying that understanding to flood management is important if we are going to live with floods, rather than always fighting them (Lund et al. 2023; Lund 2012). The traditional idea that all floods can be controlled is naïve and wrong, but remains popular and politically expedient. Repeated onslaughts of “atmospheric rivers” remind us of the vulnerability northern California to flooding, even while we rejoice at the great increase in water stored behind dams and in the mountain snowpack, as occurred this year. But what if the storms had been even bigger and more frequent, as happened in 1861-62 when the Central Valley became an inland sea (Kelley 1989)? Is existing infrastructure up to the task for ‘controlling’ such truly large floods?
Developing alternatives to traditional flood management is urgent as global warming increases the frequency and size of flooding events in waters from small creeks to big rivers and adds floods considerably larger than those previously experienced. The geologic and historic records show such extreme floods (often called megafloods) will occur and global warming makes it likely they will occur more often (Ingram and Malamud-Roam 2013; Huang and Swain 2022).
Opperman et al. (2017) presents a positive view of floods and flooding, through a review of the ecology and management of floodplains in temperate regions. They present several maximsto form the basis for a floodplain management strategy that recognizes how floods benefit both people and nature, as well as being harmful. Here we present a revised list of eleven such maxims that provide some foundational truths about floods, flooding, and floodplains in California, especially in relation to fish and wildlife populations. The maxims also need to be viewed in the context of the California water model (described by Pinter et al. 2019) which describes how the success in solving California water issues depends on “far-sighted incrementalism” and learning from failures. These maxims are good companions for further application of the California model.
1. A bigger flood is always possible. Both recent and geologic evidence support this statement. For example, the Eel and Klamath rivers experienced record flooding in 1955, which produced some huge flows. An even bigger flood occurred in 1964 when the Eel River flood crested at 46 ft (14 m) above ‘normal’ (https://en.wikipedia.org/wiki/Christmas_flood_of_1964). Records of sediment deposition in the Central Valley indicate such extreme floods have a recurrence interval of about 100-200 years (Ingram and Malamud-Roam (2013). However, as the Eel River example shows, nature does not care about average return intervals, especially with a changing baseline due to climate change. Native fishes have adaptations to survive and even thrive in big floods. Species such as Sacramento splittail and Chinook salmon have life histories that can take advantage of extended flooding for rearing of their young. But extreme floods remain a big unknown in terms of their potential effects on fish and wildlife in today’s highly altered landscape.
2. The current water management infrastructure is aging and increasingly likely to fail without major investments in repair, rehabilitation, and/or reinforcement. Effects of dam and levee failures on fish and wildlife can only be guessed at, but will depend on when and where failures occur. Dam failures are not new to California, but the recent failure of Oroville Dam spillways is a recent reminder that this can happen, especially given that California dams median age is 75 years. When dams, levees, and other types of flood infrastructure are overwhelmed by extreme flows, failure can have catastrophic consequences (Cox 2023), especially if a dam failure causes a domino effect of failures downstream. Cox (2023) notes that some dams can fail not only from deterioration but from not being designed to handle extreme events. Leaving impacts on people aside, what happens to the fish and wildlife populations with dam failures and more extreme floods? California’s inland fishes persisted through extreme floods in the past (Moyle 2002). So fish populations should continue to persist if enough individuals survive over a wide area. This means present-day conservation efforts for our declining endemic fishes must succeed before the next extreme flood arrives.
3. Flood flows shape the landscape. The bigger the flood flows, the more they will bring changes to habitats for fish and people. In California, people generally have a low appreciation for how the landscape was shaped by big floods, from the erosion of canyons in the mountains to the deposition of sediment on the valley floor, estuaries, and coasts. The floor of the Central Valley reminds us that extreme floods tend to even out this landscape, creating a flat topography interlaced with drainage systems, sloughs, and streams. This topography allowed construction of the fields that support productive farms and floodable habitats for fish and wildlife. Extreme flows should thus be expected to alter the landscape, eroding sediment here, depositing sediment there, carrying some into estuaries. If there is insufficient room for this action in existing floodplains and bypasses, big flows will create room by washing away levees, eroding farmland, enlarging sloughs, recreating drainageways and other processes. When “record” flood flows come again, providing Room for the River ( see#5) will be regarded as genius. The native fishes will appreciate it as well. The question becomes: Will our society have the wisdom, creativity, and resources to make room for extreme floods, even greater than the legendary 1861-62 flood of the Central Valley and southern California or the 1964 flood of the Eel, Klamath, and other coastal rivers?
4. Years between floods are the best times to reassess floodplain management. Calls to reduce the damaging effects of floods peak as flood waters recede but solving flood problems can be quickly neglected as other crises develop. The dry intervals between floods, especially if long and droughty, tend to reduce attention and investments for living with floods, including extreme floods. Non-flood years have traditionally been times to repair or expand levees and dams. But dry years also should be the time to carefully designate floodable lands and to build ecologically-friendly flood infrastructure, especially bypasses around cities. Developing ‘green’ infrastructure that allows us to better live with floods, rather than fighting them, will be a long and costly, but can pay off with reduced damage in urban areas, loss of species and habitat, and loss of human life. It can also pay off in terms of increased abundance of fish and wildlife, perhaps even reversing the declines of some species, and improved recreation and quality of human life in flood and non-flood years.
5. The best protection against a damaging flood is a large, well-managed floodway that keeps floods away from people and keeps people away from floods while providing habitat for native fish and wildlife. Levees and dams tend to give a false sense of security about being immune to flooding. Incidents like the Oroville Dam spillway failures in 2017 or repeated, if infrequent, returns of Lake Tulare can ‘shake up’ people but are quickly forgotten (https://californiawaterblog.com/2023/04/16/lake-tulare-and-its-fishes-shall-rise-again/). People like to build on cheap, flat floodplain land and expect to be protected from floods. But at some point, our society will recognize that it is less costly to purchase houses and other structures on floodable land, than to keep building bigger levees, paying for disaster assistance, and suffering flood damages (Schaefer and Sanders 2020). The Yolo Bypass is a good example of a well-managed floodway; it routes flood water away from the city of Sacramento while providing habitat for fish and wildlife. In most years, it also supports productive agriculture. The bypass is now being enlarged to accommodate larger floods and additional benefits. The future lies with creating more green infrastructure, such as designated floodplains that support wetlands and riparian forests. Levee setbacks can be part of the green infrastructure approach. This is referred to as making ‘Room for the River’ in the European literature. Increasingly floodable agricultural land is being valued in this regard as well, partly because such lands can also contribute to groundwater recharge. Traditional ‘gray infrastructure”, such as dams and levees ultimately have to be replaced or repaired at great expense. In California significant parts of this infrastructure may be past due for replacement (Cox 2023).
6. Flood management is most effective when implemented at the scale of the entire river basin or watershed. Much damage from the great floods on the Eel and other north coast rivers in 1955 and 1964 was from poorly regulated logging that destabilized geologically fragile slopes and sent huge amounts of sediment and debris down the river, destroying towns and natural habitats. Yet the massive redwood and fir trees that historically stabilized the watershed and were the object of logging, were growing most luxuriantly in sediment delivered by decades of previous floods. Forests are currently threatened by increased fire severity from a century of poor fuel management and a warming climate and although the cause is different, harmful floods are increasingly likely. Recognizing the importance of watersheds in flood management means using on a diverse portfolio of management methods that work with natural processes. Anadromous fishes such as Chinook salmon, coho salmon, and steelhead rainbow trout particularly benefit from this approach because they have life histories that use different parts of the watershed, as well as the ocean, at different times in their life cycle.
7. Floodplains can support diverse ecosystems and agriculture simultaneously. This is a basic theme of Opperman et al. (2017). Floodplains and flood-generated lakes in California’s Central Valley once supported the most productive and diverse aquatic ecosystems in the state; the ecosystems in turn supported a large population of indigenous peoples who harvested the abundant fish, wildlife, and plants. Research at UC Davis and elsewhere shows that the remnants of this natural system can be restored to some extent (e.g., Cosumnes River), although these remnants become novel ecosystems where the remaining native biota shares the resources with abundant non-native species, where the land and water are highly altered and where the ecosystem is managed by people. ‘Restored’ floodplains, such as those of the Yolo and Sutter Bypasses, can support abundant migratory fish (salmon, splittail) and waterfowl, as well as resident species (e.g., giant garter snake, river otter) in specially managed areas. The two bypasses also support diverse and productive agriculture, which can be compatible with rearing fish and waterfowl. Creating room for large-scale floods does not have to create a disaster for fish, wildlife, and farmers. Large scale floods can have benefits, if respected and understood.
8. Flood flows are important for the productivity of estuaries and near-shore marine environments. We should stop thinking that floodwaters are “wasted to the sea” (Cloern et al. 2023). They provide nutrients and sediments needed for estuarine and marine ecosystems, likely increasing the production of harvestable species. We tend to forget that flood flows connect land, river, and ocean habitats, and ocean productivity can be returned to inland waters by anadromous fishes such as salmon. Preparation for extreme floods also has benefits for improving protection from more frequent ‘regular’ flooding and providing fish and wildlife habitat as well as recreation for people in most years.
9. California’s endemic native fishes are adapted for persisting though major floods and droughts (Moyle 2002). However, it is not known if they can persist under the extreme circumstances from extreme floods following a long drought and then interacting with today’s highly altered landscape. The basic strategies of fish for surviving extreme events are (1) hunker down, (2) go with the flow, (3) be someplace else when the floods occur (e.g., at sea or in lakes), and (4) be lucky to survive but have a life history that allows for rapid re-colonization. Recognizing these strategies could allow for creation or protection of survival habitat (e.g., floodplain lakes on edges of bypasses) before extreme floods occur.
10. California’s terminal lakes require flood flows for persistence and rejuvenation. Terminal lakes occur at the bottom of large watersheds that have no further drainage. The water usually enters during high winter/spring flood events and leaves mostly by evaporation, although some lakes connect to rivers in major floods. As a consequence, many terminal lakes are quite alkaline (salty) and some cannot support freshwater vertebrates. California has many such lakes including: Goose Lake (on the Oregon border), Eagle Lake (Lassen County), Honey Lake (Lassen Co.), Mono Lake (Mono Co.), Owens Lake (Inyo Co.), Salton Sea (Imperial Co., mostly), and Tulare Lake (Tulare Co.). These are some of California’s most interesting and productive aquatic ecosystems. They function best with steady annual inflow to maintain minimum levels and periodic big floods for rejuvenation and reduced alkalinity. Each of these lakes is unique in its water chemistry, biota, and annual cycles. If a lake dries up because of diversion of inflowing water, or extended drought, the dry lakebeds can produce dust storms, with toxic alkaline dust, which make it hard for people to live close by. The dust storms can be so extensive at times they affect air quality in distant cities, such as when dust from dry Owens Lake or the Salton Sea reaches the Los Angeles region.
11. Climate change increases uncertainty on the magnitude, extent, and frequency of floods. These conditions look to be much more extreme than most Californians have experienced so far, with longer droughts switching abruptly to extreme wet conditions, as seen in the past decade (Huang and Swain 2022). Given the accelerating warming with its effects on the global hydrologic cycle, extreme floods are highly likely to be part of our near future.
Investing in green infrastructure to reduce the impacts of major floods on people and native biota is wise and far-sighted policy. Even if extreme floods remain infrequent (not likely), green infrastructure will create a more positive environment for people (e.g., open space, recreation, groundwater recharge, agriculture) and native biota (e.g., expanded wetlands and riparian forests) during the 99% of the time the land is not flooded. Such investment could also pay off for reducing the impacts of inevitable future dam and levee failures. Key investments of this kind will involve ensuring there are places where water can go during floods of all sizes. These include maximizing the extent of Lake Tulare, allowing Delta islands to flood, keeping flood-friendly farmland as farmland, and creating more bypasses around urban areas.
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Rypel, A.L. et al. 2023. What’s the problem with deadbeat dams? https://californiawaterblog.com/2023/05/28/whats-the-dam-problem-with-deadbeat-dams/
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The authors of this blog are all affiliated with the Center for Watershed Sciences at the University of California – Davis.
 A maxim is “a short pithy statement expressing a general truth or rule of conduct” – Google