Kym Kemp / @ 8:25 a.m. / Environment

Restoring the Sponge on the North Coast —A Whole Systems Approach

Remaining old-growth forest preserves such as Gilham Butte provide late season flows to both the Mattole and Salmon Creek watersheds.

Photo: Kyle Keegan

The following piece is a guest article by Kyle Keegan.  It was first printed in the Trees Foundation Newsletter. It is reprinted here with their kind permission. In a society that sees the effects of diminishing streams and increasingly damaging wildfires, the more knowledge we can gain about the systems that affect them, the more we can act to prevent some of the damage.

“…we are circling in a continuous stream of land, water, and life, and every part of it is critical to the fertility and health of the whole. Central to that flow of energy and life is the hydrologic cycle.” … Jerry Dennis


Nature has evolved patterns of design that work intimately with the hydrologic cycle, adapting methods to capture and store the life-giving energy embodied by water. The close and careful observation of natural systems offers us insights to emulate similar patterns within our human-built restoration designs, work, and settlements.

The ability of a watershed to receive, hold, and retain in-coming precipitation, while then slowly releasing that water over time to supply healthy stream flows—defines the “sponge.” Recent studies of North Coast watersheds have highlighted that human use (water consumption) may not be the only factor causing low flows in area streams and rivers, suggesting that the land’s capacity to catch and store water has been compromised. In essence, we are losing the sponge.

In order for a watershed to function as a whole, every function must be supported by many elements. The integrity (quality and quantity) of water within an ecosystem is reinforced by the structure, diversity, and stability of that particular system. As we begin to understand the complexity of natural systems and their interwoven parts, we come to realize that subtle tweaks to the system, or marginal changes in our interactions with the system, will not be sufficient to restore damaged processes. Thus, a whole systems approach will be necessary in restoring hydrologic functions to the North Coast. 

In this article I will describe several key ecological processes/elements that are linked to healthy stream flows, while suggesting that we may not be able to “pick and choose” which processes/elements are most critical to protect or save in our conservation efforts; since most, if not all, have hidden links to vital ecosystem services that we have yet to understand. The primary focus will be on forest ecosystems and their effects on hydrology prior to Euro-American contact. Emphasis is placed on upper watershed processes and their role in receiving and retaining water for groundwater recharge.

This will be the first in a series of three articles. Article #2 [to appear next week in the Trees Foundation Newsletter] will cover the legacy of past and present impacts and their detrimental effects to the “sponge.” Article #3 will focus on “system-based” restorative design and implementation strategies for recovery.



Soil and forest duff woven together by mycorrhizal threads
Photo: Klye Keegan

Ancient Temperate Rainforests—A Living Organism

The Ancient Temperate Rainforests of the North Coast were once part of a large contiguous expanse of living soil, plant, and animal diversity that spanned from Northern California to the Gulf of Alaska (visualize one massive green organism 100 miles wide and 2500 miles long). Over millennia, this robust living system had become so substantial that it could moderate its own climate, buffer the effects of powerful Pacific storms, and withstand the continued onslaught of fires, floods, and disease.

Intact forests once served as the living conduit between sky and land, capturing and channeling energy (sunlight, carbon, water, and nutrients) into the surrounding ecosystem, while building a complex foundation of stability for the species it supported and was supported by. The array of ecosystem services provided by the forest served to moderate and nurture its own environment, providing the very conditions that perpetuated the life of the forest itself.

The ability of forests to capture and store energy (sunlight) through photosynthesis allowed entire systems to accrue matter (biomass) both living and non-living, over time. A substantial portion of that energy captured was deposited into the building of still more complexity (biodiversity), which helped to ensure the long-term survival of the forest ecosystem. In other words, the forest’s survival depended on the investment of time and energy into its surrounding community.

Creators and Protectors of Water

“In the U.S (excluding Alaska) two thirds of water run-off comes from forests.” … David A. Perry


The Yellow Spotted Millipede (Harpaphe haydeniana) is considered a “keystone” regulator of nutrient cycling in conifer forests, just one example of a hidden link to the “sponge.”
Photo: Kyle Keegan

The conventional wisdom of cultures living close to the land is that forests are the creators, protectors, and providers of clean water. Water vapor that is kept in circulation creates the potential for more rainfall. For example, forests of the Amazon Basin are estimated to create over half of their own precipitation from evapotranspiration alone. Paradoxically, transpiration loss from trees can have a profound effect on stream flows. The structure, efficiency, and climate moderating effects of old-growth forests allowed them to conserve soil-moisture and reduce transpiration loss. (We will learn in Article #2 how young forests are adversely affecting stream flows.) Ancient trees have been found to transpire significantly less than younger trees, suggesting that like a wise elder, mature trees exert control.

On the North Coast, the hot dry summers of a Mediterranean climate interacting with the cold waters of the Pacific Ocean created abundant fog. The rough textured structure of old-growth coniferous trees were able to harvest this fog by literally “raking” the moisture out of the air. This moisture would then coalesce into canopy-induced rain, supplementing yearly precipitation during the driest parts of the year. The abundance of fog also reduced evapotranspiration rates of the forest, increasing stream flows.

Ancient Forests and Falling Rain

“Every tree, every plant species, intercedes in rain to change
the composition, energy, and distribution of water; the overall
effect of trees is to moderate and conserve incoming energy.” … Bill Mollison

The upper canopy of old-growth forests interrupted the energy of heavy rains, reducing the erosive forces on the land. As precipitation continued downward, it entered into the mid forest canopy. This realm was comprised of lush, epiphytic gardens festooned with bryophytes (mosses, hornworts, liverworts) and lichens. Bryophytes can hold 1.5-15x their weight in water and ancient trees average 80 lbs of bryophyte and lichen biomass (dry weight) per tree. When saturated, up to 145 gallons of water can be temporarily stored in the canopy of a single old-growth tree. One might hypothesize that on a landscape scale, this moisture laden bryosphere could moderate humidity levels (vapor pressure) in the forest canopy, signaling the trees to reduce evapotranspiration rates. A recent study found that certain species of canopy bryophytes are colonized by nitrogen-fixing cyanobacteria, helping provide trees with nitrogen, an element that is often lacking in forest soils.

Much of the rain that falls in forested landscapes is intercepted by vegetation and lost to evaporation. Rain that does make it to the forest floor is called “through-fall”. Due to the open and spacious structure of old-growth forests, through-fall rates are as much as 2x higher than in younger forests.

Living Soil as a Sponge

“Foresters, ecologists, and managers are recognizing now that forest productivity, recovery, and stability depend on organisms and processes below-ground.” … Michael P. Amaranthus

Healthy soil forms the basis of productivity in all terrestrial ecosystems, yet the life in soil and its effects on our environment are perhaps the least understood of all Earth’s biological processes. Soil houses an estimated 95% of the Earth’s terrestrial biodiversity, while containing 3x the amount of carbon that is held in aboveground vegetation. On average, an inch of topsoil can take between 800-1000 years to be formed. The thin layer of living soil that once covered the entire North Coast region provided the foundation of innumerable processes for terrestrial and aquatic organisms.

Ancient forests accumulated a thick absorbent layer of forest duff, insulating the living soil from weather extremes, while reducing moisture loss. Holding the forest duff in place was a filamentous mat of fungal strands (mycorrhizae) that formed a living-web of erosion control. Beneath the layers of forest duff existed a complex subterranean ecology, teeming with an unimaginable cosmos of microbial life. This underground community worked closely with the surrounding forest ecosystem, co-evolving mutually beneficial relationships that had developed over thousands of years.


Watershed Diagram: The dendritic patterns of watersheds remind us that water and forests are one.
Photo: Drawing by Kyle Keegan

Trees supply the energy for soil life by converting carbon dioxide (co2) through photosynthesis into a carbohydrate-based high energy food. This food is made available to the soil community through a nectar-like substance exuded from the trees roots; the energy rich nectar supplies the power for soil life—the soil life provides the services necessary for the trees.

An unimpaired soil community works symbiotically with the forest, increasing the efficiency of water and nutrient uptake. Elaine Ingham, an acclaimed soil microbiologist states, “a healthy soil ecology can reduce plant’s water needs by up to 70%.” This symbiosis and mutualism may explain at least one link to the reduced water needs of old-growth trees.

The young geologies of North Coast forest lands tend to have high porosity, permeability, and a gravel-like structure, i.e., they drain quickly. The “sponge-effect” was created in part, by vast underground networks of mycelial threads (hyphae) that bound soil particles; forming aggregates which created a favorable soil architecture for water retention. Recent studies have shown that glomalin, a glue-like substance produced by mycorrhizal fungi, is responsible for the majority of carbon (up to 40%) that is present in soil. Glomalin’s glue-like properties form aggregates that can persist in the soil for decades. Humus (another form of stable carbon) which is abundant in old growth forests, can hold up to 4x its weight in water.

In a healthy forest ecosystem, the tight cycling of water and nutrients between trees and the living soil creates what ecologists call a “biological dam”. This means that the wealth of the ecosystem is stored (immobilized) and continuously recycled between soil organisms and the surrounding forest community, so long as no major disturbance disrupts the cycle. It’s through this process that the structure of the sponge is developed over the course of centuries. Hence, it’s the soil life that creates the sponge and it’s the forest that creates and maintains the conditions conducive to that soil life.

Large Woody Debris

The large woody debris (decaying logs) supplied by ancient forests had a strong influence on hydrologic processes. Wind storms, land slides, and occasional stand-replacing fires, helped to deliver an abundance of large woody debris to the forest floor and stream channels. Giant decaying logs laying on the ground absorbed and held copious amounts of water, providing a source of moisture to forests during late summer months.

Fallen logs that entered streams created in-stream terraces or check-dams which modified the hydrologic energies and gradients of water courses. These check-dams slowed water down, stored sediment, captured organic material, increased channel complexity, and elevated the water table acting as small high elevation reservoirs that slowly released water back into the stream course, thus lengthening the hydrologic cycle. The quantity of water that could be held by these natural dams depended on the amount of sediment being stored behind the woody debris, as well as the subsurface geology adjacent to the area that could facilitate the lateral movement of water into the surrounding landscape. Water that passed through these natural dams was then cooled (refrigerated) after flowing through subsurface sediment layers, to later return downstream.

Leave it to Beaver

The vision of “restoring the sponge” cannot be told without reflecting on the beaver’s (Castor canadensis) role in North Coast ecosystems. Almost every northern temperate ecosystem that had trees or shrubs growing along streams also once had beaver dams. Although the exact locations of past beaver populations on the North Coast is uncertain, what we do know is, where the beaver has returned, so have the stream flows.

As Nature’s best buck-toothed engineers, beavers built dam complexes that: increased groundwater recharge, elevated the water table, attenuated stream discharge rates, provided biological filtration, enhanced riparian plant communities, and created indispensable habitat for amphibians, neotropical birds, aquatic invertebrates and salmonids.

Forests Built of Salmon

As a “keystone species,” salmon built the forests, which in return, sheltered the salmon. Stable isotope studies have shown that abundant salmon runs once delivered an astounding supply of marine fertility (nitrogen, phosphorous, potassium, and calcium) to freshwater and terrestrial ecosystems along the Pacific Coast; a process that biologists now call the “Anadromous Nutrient-Pump.” The bodies of salmon in the tens of thousands were once carried both physically and in the bowels of grizzly bears, black bears, bald eagles, condors, coyotes, raccoons, and humans up into the most remote reaches of watersheds to be reincarnated into other living beings.

The return of salmon can be seen as the critical link to the health of forests—which in turn feeds the living soil—that creates the sponge—that provides the stream flows—which allow the salmon to return—to feed the very trees—that once sheltered the salmon.

Indigenous Knowledge of Fire and Stream Flows

Frequent low-intensity ground fires played an important role in the health, maintenance, and diversity of North Coast ecosystems. The use of prescribed fire by the indigenous peoples of the North Coast helped to maintain the understory of forests, keeping the landscape clear of brush and young trees while also keeping native prairies open. The removal of brush and young trees reduced the evapotranspiration rates of landscapes, thus increasing late season stream flows. In Kat Anderson’s book, “Tending the Wild,” Rosalie Bethel, a North Fork Mono describes, “Both men and women would set the fires. The flames wouldn’t get very high. It wouldn’t burn the trees, only the shrubs. They burned around the camping grounds where they lived and around where they gathered. They also cleared pathways between camps. Burning brush helped to save water.”

The knowledge of fire and its effects on the hydrologic cycle were most understood by the original inhabitants of this region. Frank Lake, research ecologist writes, “Over millennia, tribal people developed an understanding of the relationship between the extent and severity of fires, change in vegetation composition, structure and function, modified (reduced) transpiration levels, affected (increased) spring flows/hydrology, and aquatic habitat quality to support healthy and productive fisheries.”

Frequent intervals of low-intensity ground fires also helped to prevent the possibility of high-intensity stand-replacing fires by reducing fuel loads. Prescribed fire not only augmented stream flows, it also provided a certain level of security against catastrophic fires, increasing the resiliency of the forest ecosystem.

Perennial Grasses and the “Sponge Effect”

In the foothills and on slow moving earth-flows and clay soils, oak woodlands and native prairies dominated the landscape. These soils are much different from the steep, gravelly soils that sustain conifer forests in the upper catchments. Highly sheared clay soils are slow to receive water, but after becoming saturated, they hold water for longer durations.

The oak woodlands and prairies of the North Coast once contained an unparalleled diversity of herbaceous plants, wildflowers and perennial bunch grasses. The perennial bunch grasses of prairies and oak woodlands had co-evolved with fire and were suited to survive the prolonged dry summers of a Mediterranean climate. Some perennial bunch grasses are extremely long-lived, with species such as Purple Needle Grass (Nassella pulchra) living hundreds of years. Long-rooted perennial grasses allowed for the percolation and infiltration of water deep into the soil horizons. (Perennial grasses can have root systems 6-18’ deep.) Through the oscillations of growth and decay imposed by droughts, fire, and herbivore browsing, the long tap-roots of perennial grasses stored carbon (humus) in the depths of the soil horizons, creating a sponge-like effect, which in turn elongated the hydrologic cycle.

Massive flocks of Band-Tailed Pigeons numbering in the thousands would arrive each Fall to gorge on acorns, dropping their fertile guano in exchange, and each spring would bring the return of countless neotropical migrant birds to forage on the abundance of insect biomass provided by the oak canopies—both examples of what could be called the “Avian Nutrient-Pump”.


Remnant perennial bunchgrass prairie, (Festuca californica) and (Danthonia californica) in the Salmon Creek Watershed
Photo: Kyle Keegan

The health and resilience of these systems were co-managed by both human and nonhuman beings for millennia prior to Euro-American contact. One could theorize that the hydrologic capacity of native prairies and oak woodlands to receive, store, and slowly release water was maintained by the knowledge of indigenous peoples and their use of fire, biodiversity, and the abundance of long-lived perennial grasses.

Towards the Ocean

Over the course of thousands of years, the low-gradient reaches of watercourses accumulated a wealth of alluvial and sedimentary deposits. These expansive floodplains, rich in carbon and mineral fertility, provided a natural reservoir for groundwater. Vast stands of riparian forest (redwood, alder, willow, and cottonwood) served to: dissipate flood energies, moderate peak flows, recharge aquifers, capture nutrients, provide habitat, nourish macro invertebrates, and shade stream reaches. Natural meanders and deep channels with abundant large woody debris further helped to slow down flows, while providing in-stream habitat and cover to salmonids on both their upstream and downstream journeys.

Towards the end of the water’s travels (or shall we say the beginning), estuaries, marshes, and wetlands provided the final cleanse and capture of nutrients. This dynamic ecosystem served as both the “liver” and the “nursery”; raising and sheltering a myriad of species before allowing the passage of water to the sea—to be cycled once again by the storms of the Pacific—returning as rain to the lands of the North Coast.

Water is Life and Life is Water

“Water is life,” is the age old proverbial saying. Since in Nature, energy flows both ways, we can also say that “life is water.” Being that living organisms by volume are mostly water, we can theorize that the capacity of a terrestrial ecosystem’s potential to capture and store water is directly proportionate to the capacity of that ecosystem to support life.

Download this article, with all of its citations, in PDF format

In the next article we will learn how disrupting that life/water cycle can have a profound impact on North Coast communities.  

Kyle Keegan has lived with his family in the Salmon Creek watershed for the past 15 years and has been actively involved in restoration, environmental education, and local issues pertaining to land stewardship. Kyle can be reached at




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