Riverbanks are transitional boundaries, or ecotones, between
the aquatic and terrestrial ecosystems, and they frequently
change under naturally dynamic hydrologic conditions.
Although abundant evidence suggests that bank erosion is a
necessary ecological process (Piegay et al. 1997, 2005), current
river management, and sometimes even restoration strategies,
calls for channel bank infrastructure, that is, hard structural
elements intended to arrest bank erosion (also called revet-
ment, erosion control, or bank stabilization structures). Such
strategies often focus on human values that include property
damage and land loss, flood hazards (Piegay et al. 1997,
Casagli et al. 1999), and potential impacts to aquatic habitat
from bank-derived fine sediment contributions (EPA 2007).
Often, projects labeled as “restoration” focus principally on
bank stabilization. However, static banks are not the norm,
and static rivers and streams do not sustain ecosystems.
Despite this, in response to the notion that bank erosion is
deleterious, the construction of bank infrastructure has
become pervasive over the past century as an increasing pop-
ulation and associated development encroach on riparian
landscapes. Thus, bank erosion management is a significant
In this article, we review the ecological significance of
a range of geomorphic bank erosion processes and show
that the cumulative effect of progressive bank stabilization
structures is to limit riparian function and diminish habitat
for riparian species. Our objectives are to (a) synthesize geo -
morphic and biological literature through principles that
highlight the importance of bank erosion processes as dis-
turbances integral to components of riparian ecosystems at
ank erosion is a natural geomorphic process or
disturbance that occurs during or soon after floods.
a variety of scales; (b) identify the effects of channel bank infra -
structure on riverbank and riparian ecology; (c) identify fail-
ures of current policies to manage channel bank erosion;
and (d) present a rationale and framework for alternatives to
such policies. The alternatives are intended to aid the devel-
opment of river management and policy that promote health-
ier geomorphological and ecological functions in river systems
where bank erosion is an issue of concern.
Geomorphic and ecologic significance
of banks and bank erosion
We define “riverbank,” in a geomorphic context, as the land-
form distinguished by the topographic gradient from the
bed of a channel along the lateral land-water margin up to the
highest stage of flow or up to the topographic edge where
water begins to spread laterally over the floodplain surface.
Bank erosion refers to the erosion of sediment from this
distinct landform. Eroded sediment moves along the topo-
graphic gradient laterally toward the channel or in the down-
stream direction. Banks are often characterized by bare
sediment, live vegetation, or snags (Roy et al. 2003). In an
ecological context, riverbanks are an important component
of riparian zones. Bank habitat and function are to some
degree inseparable from functions within the larger riparian
zone; here we take a broader view of natural banks and
bank erosion as they influence riparian areas. Ecologically
Joan L. Florsheim (e-mail: firstname.lastname@example.org) and Jeffrey F.
Mount are with the Department of Geology at the University of California at
Davis. Anne Chin is with the Department of Geography at Texas A&M Uni-
versity in College Station. © 2008 American Institute of Biological Sciences.
Bank Erosion as a Desirable
Attribute of Rivers
JOAN L. FLORSHEIM, JEFFREY F. MOUNT, AND ANNE CHIN
Bank erosion is integral to the functioning of river ecosystems. It is a geomorphic process that promotes riparian vegetation succession and creates
dynamic habitats crucial for aquatic and riparian plants and animals. River managers and policymakers, however, generally regard bank erosion
as a process to be halted or minimized in order to create landscape and economic stability. Here, we recognize bank erosion as a desirable attribute
of rivers. Recent advances in our understanding of bank erosion processes and of associated ecological functions, as well as of the effects and failure
of channel bank infrastructure for erosion control, suggest that alternatives to current management approaches are greatly needed. In this article,
we develop a conceptual framework for alternatives that address bank erosion issues. The alternatives conserve riparian linkages at appropriate
temporal and spatial scales, consider integral relationships between physical bank processes and ecological functions, and avoid secondary and
cumulative effects that lead to the progressive channelization of rivers. By linking geomorphologic processes with ecological functions, we address the
significance of channel bank erosion in sustainable river and watershed management.
Keywords: bank erosion, riparian ecology, fluvial geomorphology, sediment, aquatic ecology
www.biosciencemag.orgJune 2008 / Vol. 58 No. 6 • BioScience 519
by guest on December 26, 2015
functioning riparian zones provide a variety of resources
and are vital centers of biodiversity (Gregory et al. 1991,
Naiman et al. 1993, 2005, Ward and Tockner 2001, NRC
2002). The main functions of riparian zones are related to
fluvial hydrology and sediment dynamics; retention and
cycling of nutrients and pollutants; and maintenance of habi-
tat for wildlife, including invertebrates, amphibians, reptiles,
birds, and mammals (NRC 2002). In the following sections,
we review elements of banks and bank erosion that create
physical and biological heterogeneity and riparian diversity.
We focus discussion of bank processes and functions around
principles that illustrate the significance of bank erosion and
natural banks as desirable attributes of rivers:
• Bank erosion provides a sediment source that creates
• Active banks create and maintain diverse structure and
• Riparian vegetation promotes bank stability and con-
tributes large woody debris.
• Bank erosion modulates changes in channel morpholo-
gy and pattern.
Channel banks form a significant ecotone between aquatic and
terrestrial ecosystems with diverse structure and habitat func-
tions; this article forms the critical basis for discussions of the
effects of and alternatives for channel bank infrastructure.
Bank erosion provides a sediment source that creates riparian
habitat.Diverse bank erosion processes occur as sediment cy-
cles through the continuum of headwater to lowland envi-
ronments within a watershed (figure 1). The dominant bank
erosion process in each part of the watershed is influenced by
the size of the channel, discharge, and flow strength (Couper
2004), with the dynamic nature of erosion processes de-
pending in part on sediment supply and transport regime
(Benda et al. 2004). Fluvial deposits vary dynamically from
the headwaters to the lowland (Church 2000). Bank erosion
from headwater areas provides a source of weathered sediment
that is stored for varying periods in downstream alluvial de-
posits (Gomi et al. 2002).
Bank erosion is a considerable sediment source in some
rivers (Trimble 1997); however, the sediment supply is not al-
ways deleterious. Bank erosion supplies coarse sediment to
channels—a size fraction that is necessary to form the phys-
ical structure of aquatic habitat. Coarse sediment, supplied
from upstream and stored as channel-bed material and bed-
forms, makes up substrate important for macroinvertebrates.
Such coarse-grained substrate promotes oxygen exchange, pro-
vides interstitial space for protection from predators, serves
as attachment sites for filter feeders, and provides a food
source for periphyton (Wood and Armitage 1997). In contrast,
when the sediment supply is large relative to transport capacity,
such that aquatic habitat is buried, or when fine-sediment con-
tributions from bank erosion are excessive, habitat damage
may occur. In streams with large sediment inputs derived from
bank erosion, there is often concern that changes in water
quality due to large fine-sediment loads affect aquatic
habitat (EPA 2007). Large fine-sediment inputs may affect
groundwater-surface water exchange, a factor in fish and
benthic invertebrate habitat (Lisle 1989, Kondolf et al. 2006).
Processes that include infiltration of fine-grained sediment into
coarser channel substrate may in turn impede intergravel
water flow in the hyporheic zone, consequently reducing
oxygen levels to benthic organisms.
As a physical process that supplies and delivers sediment,
bank erosion is critical for creating habitat at the watershed
scale (figure 1). Riparian area structures are influenced by
variations in geomorphic processes and in the resulting
valley bottom deposits, including floodplains and bars
(Gregory et al. 1991). Floodplain ecosystems, a critical com-
ponent of riparian ecosystem diversity (Ward and Stanford
1995, Stanford et al. 1996), are sustained by periodic erosion
and sedimentation during floods (Junk et al. 1989, Bayley 1991,
1995, Florsheim and Mount 2002). Bank erosion also con-
tributes sediment to fluvial deposits, such as sandbars in the
Platte River, that are important to migrating whooping cranes
(Grus americana). Resting on the bars during their migration,
these birds have long sight lines and are isolated from preda-
tors (NRC 2002, Graf 2005).
520 BioScience • June 2008 / Vol. 58 No. 6www.biosciencemag.org
Figure 1. Illustration of a river network from headwater
to lowlands. Bank erosion is one component of the sedi-
ment cycle throughout an idealized river network. In the
headwaters of watersheds, banks are the boundary be-
tween upland terrestrial and aquatic ecosystems. In low-
land areas, channel banks are commonly the transitional
area between floodplain and aquatic habitats. Sediment
eroded from hill slopes in headwater areas is transported
downstream and stored in deposits (such as terraces,
floodplains, bars, and channel substrate) that provide
habitat for aquatic and riparian organisms.
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