Sedimentation in California Reservoirs: a Long-term Problem of Immediate Concern

• Jurisdiction: California,United States
• Author: by Ryan J. Mahoney
• Publication year: 2019
• Citation: Vol. 28 No. 2

Sedimentation in California Reservoirs: A Long-Term Problem of Immediate Concern

by Ryan J. Mahoney^*

Ryan J. Mahoney

I. INTRODUCTION

Sedimentation in reservoirs is a threat to California's water supply, water quality, environmental health, and public safety.¹ The accrual of sediments on reservoir floors reduces water storage capacity, causes the heavy concentration of toxic mate-rials, and negatively impacts water supply reliability, reservoir infrastructure, and ecosystems in and around reservoirs.² Despite so many problems arising from sedimentation, sedimentation can be successfully removed, reduced, and eliminated with the adoption of a strong legal framework, strategic prevention and removal plans created using current and reliable data, and sedimentation management plans developed for individual reservoirs based on specific need.³ Water is too important for California to continue to do so little to address reservoir sedimentation.⁴ California's stability, health, and prosperity are all tied to its water, but the State's reservoir infrastructure is aging and if nothing is done to address sedimentation the loss of water storage capacity, degradation of water supply reliability, and threats to reservoir infrastructure will continue and grow more severe.⁵

This article proposes that California should adopt new laws and regulations to require specific action be taken by reservoir owners, managers, and operators to prevent, eliminate, and reverse sedimentation in existing and future reservoirs.⁶ Developing laws and regulations to address sedimentation will be a complex endeavor that must involve significant public investment and stakeholder input through public workshops, meetings, and hearings. Enacting these laws and regulations is imperative despite the long and challenging process required, because doing so will spur the creation and implementation of effective and efficient plans and strategies to deal with sedimentation. Sedimentation is a long-term problem of immediate concern, that California can and should address now, not leave for future generations.

II. WHAT IS RESERVOIR SEDIMENTATION

A. The Process of Sedimentation

Sedimentation is a process by which the bottom of a reservoir fills with large and small sediments transported into the reservoir by water.⁷ Dams impede the flow of water allowing sediments suspended in, moved by, or dissolved in water to settle and fall to the reservoir floor.⁸ Sediments on the reservoir floor accumulate creating highly compacted sedimentation.⁹ Highly compacted sedimentation forms, because larger more coarse sediments settle down on the reservoir floor leaving openings which smaller less coarse sediments fill.¹⁰ In summation, sediments, such as dirt and debris, that enter a reservoir, are trapped by the reservoir dam, and fall to the reservoir floor becoming tightly compacted.¹¹

B. Sedimentation—Natural and Influenced by Human Conduct

Sedimentation is a natural process in water systems without human influence.¹² However, most water systems in California are influenced by human conduct, often in a multitude of ways, two of the most common of which are dams and reservoirs.¹³ Dams and reservoirs create a form of sedimentation not found in waterways uninfluenced by human activity, because dams prevent significant portions of sediment from exiting reservoirs and flowing to ecosystems downstream.¹⁴ Additionally, many of the most significant factors impacting sedimentation are caused by human conduct, including: the amount of sediment produced in a watershed; reservoir age and condition; the efficiency at which a reservoir traps sediment; and the amount of sediment "trapped by other dams in the watershed."¹⁵ Thus, sedimentation is a natural process, but dams, reservoirs, and other human activities influence why, when, where, and how sedimentation occurs.¹⁶

III. WHY SEDIMENTATION OF RESERVOIRS IN CALIFORNIA MATTERS

A. Detrimental Impacts of Sedimentation in Reservoirs 1. Water Supply, Water Quality, and Toxic Contamination

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Sedimentation detrimentally impacts water supply, because it reduces reservoir storage capacity as sediments accumulate at the floor of the reservoir causing the floor of the reservoir to rise in elevation.¹⁷ This loss of reservoir storage capacity is detrimental, because the loss is essentially permanent since sedimentation removal is not a common practice.¹⁸ The permanent loss of reservoir storage capacity is a danger for California, because reservoirs need to maximize potential storage capacity, not only for water supply and water quality, but also for flood and environmental protection.¹⁹ The amount of available reservoir storage capacity impacted by sedimentation depends on a multitude of factors, including: the depth of the reservoir, the initial design capacity of the reservoir, the rate at which sediments enter the reservoir, and upstream influences.²⁰ Water quality is impacted by sedimentation, because sedimentation concentrates toxic chemicals, pesticide residues, and heavy metals on the reservoir floor, contaminating the water and complicating sedimentation removal strategies.²¹ No reservoir is immune from the detrimental impacts of sedimentation, therefore sedimentation should be addressed to minimize its impacts on water supply, water quality, and toxic contamination.²²

2. Reservoir Infrastructure and Economic Impact

Infrastructure is detrimentally impacted by sedimentation because sedimentation can impede the physical operation of reservoir dams by causing damage to turbines or water release outlets.²³ Additionally, sedimentation adds to the force and weight dams must be capable of supporting because sedimentation exerts significance force against dams.²⁴ Economic concerns arise with sedimentation because sedimentation reduces water supply, causes toxic contamination requiring water treatment, and results in the need for costly prevention and removal projects.²⁵ Some of the commonly explored options for addressing the impacts of sedimentation include: methods designed to avoid sediments from entering reservoirs; sediment removal through dredging or sluicing projects; and reservoir expansion projects when reservoir storage capacity drops below a specified level.²⁶ Thus, sedimentation detrimentally impacts reservoir infrastructure and causes economic concerns that are unavoidable and difficult to mitigate without an effective sedimentation strategy.²⁷

B. Detrimental Effects on Ecosystems 1. Toxic Sedimentation

Sedimentation affects aquatic and terrestrial ecosystems within reservoirs and those downstream in a multitude of ways.²⁸ High concentrations of toxic chemicals, pesticide residues, and heavy metals trapped by reservoirs cause harm to ecosystems within reservoirs, because they negatively impact both water quality and the health of plants and animals occupying or utilizing the reservoir.²⁹ Downstream ecosystems are impacted by toxic sedimentation, because sediments that have high levels of toxic contaminants are sometimes released from reservoirs causing them to enter into downstream ecosystems.³⁰ Therefore, the concentrations of toxins in sedimentation can result in harm to ecosystems within and downstream of reservoirs.³¹

2. Downstream Erosion

Sedimentation harms downstream ecosystems because it results in less replenishment of soil and rock.³² The reduction in sediment rejuvenation increases erosion in downstream rivers, creeks, and streams.³³ This erosion of banks and floors of rivers, creeks, and streams causes waterways to become straighter, deeper, and wider.³⁴ These impacts cause the reduction in available habitat for fish and other wildlife in downstream ecosystems and the loss of economically valuable riparian land.³⁵ Thus, sedimentation causes harm to downstream ecosystems in a number of ways.³⁶

3. Sedimentation Releases (Sluicing)

Sluicing is the physical release of sediments that have accumulated on reservoir floors.³⁷ Removing sedimentation through sluicing is not a common practice for several reasons, not the least of which is that it requires a significant amount of water.³⁸ Additionally, many dams were not built to effectively address sedimentation by sluicing because sedimentation occurs in areas where sluicing is not possible, dam release outlets are not in the right locations for sluicing, or the dam and its release outlets are not properly designed for sluicing.³⁹ Furthermore, sluicing can result in the contamination of the water in the reservoir and the water released in the sluicing process due to the toxic materials in the sedi-ment.⁴⁰ For these reasons sluicing has not historically been a common way to reduce sedimentation.⁴¹

C. Detrimental Effects of Sedimentation on Public Safety

Public safety is detrimentally effected by sedimentation because sedimentation causes stress to reservoir infrastructure, causes downstream waterways to erode unnaturally due to the lack of sediment replenishment, spurs human development in traditional flood plain areas, and reduces water supply capacity.⁴² Engineers dealt with sedimentation primarily by sizing reservoirs to account for anticipated sediment accumulation during the reservoir's lifetime.⁴³ Engineers expected that a certain percentage of a reservoir would fill with sediment, thereby reducing the reservoir's storage capacity by an acceptable amount during the reservoirs anticipated lifetime.⁴⁴ The increased reservoir capacity method of dealing with sedimentation resulted in most reservoirs in California not having lower dam outlets to release built up sediment on the reservoir floor or effective sedimentation prevention plans.⁴⁵ This failure to effectively plan for sedimentation has resulted in sedimentation negatively impacting reservoir infrastructure due to the stress and force sedimentation exerts on reservoir infrastructure.⁴⁶

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The changes sedimentation causes to downstream rivers, creeks, and streams impacts public safety because downstream waterways erode as...