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  • Ethan Olson

THE ROCK-DROP:

Harnessing an Indigenous Land Management Technique to Adapt to Climate Change in San Diego


BY ETHAN OLSON

 

INTRODUCTION

The San Diego of today is far different from the San Diego of 50 years ago, and the San Diego of 50 years ago was far different from the landscape that occupied its modern boundaries 400 years ago. One important difference? It’s warmer. The air is filled with more carbon dioxide, and the climate is changing. Flood risk is increasing and sensitive habitats are struggling to survive.


However, if we look back to the time when the original caretakers of this land — the Kumeyaay people — were the only humans who inhabited it, we would see an even more radical difference, a completely different landscape filled with oak woodlands, rich chaparral understories, and flowing creeks in the region’s many canyons.


One technique that the Kumeyaay used to manage this landscape sustainably is called the rock-drop technique, which involves placing large boulders into a section of a fast-flowing, highly-eroded, and largely impermeable streambed to slow down water, recharge water tables, and create healthy riparian habitats.


images from the National Museum of the American Indian


In the contemporary era, such a traditional technique could be one answer to the question of addressing climate change at the local level. In this report, I will argue why.

 

THE CHALLENGE

Climate Change in San Diego


According to climatological research by climate scientists at the Scripps Institution of Oceanography, there are several effects climate change will have on San Diego and other coastal regions across the western United States. Key among them is that precipitation frequency is expected to decrease, with a higher percentage of annual rainfall occurring within a smaller number of individual precipitation events. Essentially, this means fewer, yet stronger storms, which is not good news for a city that already struggles with flash floods on a regular basis.


Concentrating more of the region’s annual rainfall into a smaller number of days also means that a smaller proportion of this water will be absorbed into the soil. Instead, due to the intensified nature of these weather events, more water will simply become runoff that makes its way back into the ocean. The effects of this on the native ecosystems of San Diego’s canyons could be drastic.


Finally, San Diego is not exempt from the impacts of the urban heat island effect, wherein urban cores experience higher temperatures than outlying areas due to the built environment’s tendency to absorb and re-emit the sun's heat more than natural landscapes, which will become more dangerous as global temperatures increase. Low-income communities are particularly vulnerable, due to reasons such as a lack of tree canopy coverage, higher proportions of heat-absorptive surfaces such as asphalt, and lower access to air-conditioned spaces.


The Urban Ecologies of San Diego Canyonlands



As seen in the image above, San Diego is topographically unique when compared to other global cities thanks to its expansive network of urban canyons, which have remained largely undeveloped as the flat mesas of the city urbanized. The result of these conditions is a complex geography of urban-ecological interfaces that provide the city with challenges and opportunities. On the one hand, the close proximity (and often, overlap) of urban and ecological systems results in problems for the functioning of both entities. Urban systems interfere with the health of the canyon’s ecological systems via means such as the encroachment of invasive species, habitat fragmentation, and impermeable drainage channels. Meanwhile, ecological systems can threaten urban systems via floods, erosion, and dangerous wildlife.


Thankfully, however, the interaction of these two systems does not need to be fundamentally antagonistic. Ecological systems provide many benefits to the city by filtering water, stabilizing microclimates, and offering recreational opportunities to local residents. In turn, urban systems can be designed and altered to help ecological systems function through means such as the installation of green infrastructure and reforestation initiatives. In the next section, this report will demonstrate how such human interventions in the landscape are in fact not new to the region, but instead rooted in thousands of years of Indigenous history.


Kumeyaay Knowledge: The Rock-Drop


The native people of the greater San Diego-Tijuana region are the Kumeyaay — and contrary to popular belief that Native Americans simply hunted and gathered to survive — the true history of these people involves the careful management of the landscape through interventions such as rock-drops, fire mosaics, and transplanting trees:


“For Tribes, cultural prosperity is dependent on caring for the natural world. Recreating past conditions requires an understanding of how people lived in their environments…. Other practices, such as rock-drop structures enhanced groundwater recharge, stabilized stream flows, and created riparian habitat. The managed environment provided foods, medicines and building materials for the Tribe. Removing people from the landscape is neither healthy nor sustainable. Tribes have centuries, even thousands of years of experience in observing, evaluating and researching ecosystem conditions and management approaches” (Tribal Water Summit Proceedings, 2013).


The land management techniques of the Kumeyaay were abruptly stymied by the invasion of Spanish colonists, who stole land from Indigenous groups and immediately set about transforming the landscape through irresponsible cattle grazing:


“Water management, (or mismanagement) characterized the transformation of the coastal ecosystems as cattle and sheep grazing was accompanied by the clear cutting of the oak forests to increase the grazing lands. Water recharge and storage were destroyed at an accelerated rate as stream channels were opened up to direct sunlight and the water temperatures rose. In addition, the native willows, cottonwoods and associated plants could not withstand the continual onslaught of open grazing. The nutrient overload of grazing animals defecating in the streams would have further added to the impacts to water supply as algae blooms ultimately create low oxygen conditions” (Tribal Water Stories, 2009).


Historic injustices are a feature of the Southern Californian landscape. Where the Kumeyaay managed the land, the Europeans exploited it. Western conquistadors brought with them a philosophy of dominion over nature, one that was upheld by Christian beliefs and continued after the transfer of land from Mexico to the United States. Thankfully, it is not too late to learn from the past — to bring back the rock-drop, and to address climate change.

 

THE PROPOSAL

Implementation


Phase 1: acquiring funding and community support


Before any rocks can be dropped, permissions and funding for intervening in municipal public space must be obtained. Funding may come from a variety of sources: general revenue from the city budget, taxes applied to surrounding properties or corporations, carbon tax, private donors, etc. For the purpose of this report, it is assumed that necessary funds are available and planning policies are altered. It is important to note that — in part due to San Diego’s lack of connected greenspace corridors (see Boston’s emerald necklace for a good example of weaving a linkage of open spaces across the urban landscape) — the implementation of the rock-drop technique will likely need to happen on a canyon-by-canyon basis. Flexibility in funding sources, community organizing, and construction details should be maintained to prevent bureaucratic impediments further down the line.


Phase 2: construction


The construction of a rock-drop begins with the selection of the site. Generally speaking, the locations of these landscape installations should be in low-lying areas of the canyon floodplain that encounter direct contact with running water during seasonal precipitation events, and which lack absorptive soils that let water seep into the ground below.


These sites should also be a safe distance from any designated hiking trails and occupy areas that do not have a large community of native plant species already established.


Rock-drops should be placed where their impact will be highest. The following images from prominent tribal leader Michael Connolly Miskwish in the 2019 San Diego Integrated Regional Water Management Plan illustrate how the careful selection of a site can dramatically improve the health of the local landscape, in this case, at the Campo Kumeyaay Reservation.

Before the placement of the rock-drop, the strong seasonal flows of the river led to eroded banks, which in turn decreased habitat potential and biodiversity. After the installation of the rock-drop however, a wetland ecosystem gradually began to flourish as more water seeped into the soil — simultaneously recharging groundwater — and erosion slowed due to the physical impediments of the boulders.


Phase 3: growth and aging


Naturally, this brings us to the third phase of implementation — which is less about actively intervening in the land, and more about passively maintaining, protecting, and appreciating the ecological effects of the rock-drops. As time progresses, erosion is halted by the rocks, silt collects, water seeps into the soil, water tables rise, vegetation returns, and animal species follow.


Benefits and Drawbacks


The benefits and drawbacks of implementing the rock-drop technique can be easily understood by categorizing them according to the last two phases discussed above.


The benefits of Phase 2 (construction) will potentially be noticeable as soon as the first rains fall after the rocks are laid into the streambed, and include flood control, water filtration, better water absorption by soils, and increases in silt deposits. The main drawback of Phase 2 is the potential disruption of habitat caused by the construction process, which may involve heavy machinery to move heavy rocks. Another drawback is found in the potential die-off of established species accustomed to drier soil conditions; however, the lack of vegetation in rock-drop-suitable sites means that such consequences are unlikely to occur.


Phase 2: construction. Images from the National Museum of the American Indian


The benefits of Phase 3 (growth and aging) are more multifaceted and occur across varying timescales in a fairly gradual manner. These include the creation of riparian habitat, increased tree canopy coverage, the reduction of the urban heat island effect thanks to the addition of more vegetation, carbon sequestration, and further water filtration and flood control due to the dense organic matter of riparian plants. What is important to note here is that these interventions in the landscape have the potential to not only adapt to climate change, but to mitigate it as well through the sequestration of carbon dioxide into the soil via new riparian vegetation. Wetland restoration is identified as a key land-based solution in the comprehensive Bending the Curve report due to its impressive ability to sequester carbon, which further demonstrates how the rock-drop technique can help San Diego achieve carbon neutrality sooner.


The drawbacks to the maturation of rock-drops include the potential submergence of nearby hiking trails, proliferation of water-loving disease carriers such as mosquitos, and invasive species encroachments that take advantage of the wetter soil conditions.

 

CONCLUSION

There are many reasons to adopt and implement the rock-drop technique in San Diego’s urban canyons, particularly when thinking about mitigating and adapting to climate change. Rock-drops have the potential to weaken floods, filter water, sequester carbon, reestablish wetland ecosystems, and regulate microclimates. But perhaps most importantly, the initiation of rock-drops and other Indigenous land management techniques offer an opportunity to restore the landscape to a form that is closer to its historic past: one without anthropogenic climate change, and one with a community of people who take pride in caring for the land.


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