Whether
for purposes of navigation, irrigation, improved drainage, flood control, power
generation, or simply to control a vital resource, rivers and streams of all
sizes have been modified in ways great and small for millennia. Channels might
be widened or narrowed, dredged or filled in, straightened or moved entirely,
but ultimately the issue is whether the resultant “fixed” stream accomplishes
what is intended or not, and whether it is stable or not.
Pioneering
hydrologist and geomorphologist Dr. David Rosgen notes the cumulative effects of
river management and mismanagement. “The effects of road construction, riparian
vegetation change, in-channel gravel mining, logging, reservoirs/diversions,
urban sprawl, and other similar developments have significantly changed flow and
sediment regimes and the boundary conditions associated with stable stream
systems,” he states. “Direct disturbance to channels by straightening, lining,
draining, raising, lowering, clearing, dredging in the name of flood control,
navigation, and other single-purpose objectives have taken a serious toll on the
physical and biological functions of our rivers.”
Environmental
issues have become increasingly urgent. “Public awareness over the last decade
has prompted federal, state, local jurisdictions, and environmental groups to
direct major efforts at preserving, protecting, enhancing, stabilizing,
rehabilitating, and restoring rivers throughout the United States,” says Rosgen.
“Society has spent the last 200 years changing landscapes; now, they want their
rivers back.”
However,
Rosgen says, despite the best of intentions, sometimes restoration projects go
awry. “Many such failures,” he writes, “are a direct result of the lack of a
clear understanding of the cause and consequence of instability. The difference
between success and failure in river restoration is often associated with the
effort expended in watershed/river assessment.”
Early
River Restoration Approaches
Professor
Richard Hey noted in the April 2006 issue of the Journal
of the American Water Resources Association
scientific approaches to river restoration dating back to the late 19th century.
What are termed regime
equations
were developed to define how elements such as flow, sediment transport, bed and
bank material, bank vegetation, and the valley slope affect the profile and
pattern of a river. “The earliest equations,” he writes, “concentrated
exclusively on the effect of discharge on channel form and are often referred to
as hydraulic geometry relations.”
Then
there are what he terms rational
equations.
Hey explains, “Rational equations offer a theoretically based alternative to
empirical regime equations for designing alluvial channels. If equations could
be specified for each process that controls flow, sediment transport, and
channel adjustment in alluvial channels, their simultaneous solution would
enable the morphology of natural rivers to be prescribed given the boundary
conditions.”
 |
Photo: Skelly & Loy |
| First Hollow Run stream prior to restoration |
The
reality, however, is that both the regime and the rational equations have
significant limitations. Hey notes, “As most of them are restricted to the
design of straight channels in a particular river environment, it raises
questions about their general utility for natural channel design. For many types
of rivers, there are no relevant equations. Even if the equations are
appropriate, there will still be some uncertainty in the predicted channel
dimensions.”
In
1994, Rosgen published his landmark “A Classification of Natural Rivers” in the
journal Catena.
This was the culmination of some 25 years of work in which he developed an
extensive fluvial geomorphologic database composed of actual field measurements
of literally hundreds of streams throughout the US, Canada, and New
Zealand.
In
this article, Rosgen described a progression of stream classification systems
that had been previously proposed. An 1899 effort was as simple as a division
into three stages: “youthful, mature, and old age.” In 1957, straight,
meandering, and braided patterns were described, as well as quantitative
slope-discharge relationships for these patterns. In 1963, elements of channel
stability and sediment transport were added. Later that decade, additional
classification systems were developed that took into account depositional
features, vegetation, sinuosity, meander scrolls, bank heights, levee
formations, and valley types. More recent systems developed in the 1980s and
1990s added elements of the form and gradient of alluvial channels, as well as
characteristics of bank and sediment materials.
Rosgen
noted that “with certain limitations, most of these classification and/or
inventory systems met the objectives of their design,” but added, “Typically,
theoretically derived schemes often do not match
observations.”
 |
Photo: Meadville Land Services |
| Before stream restoration was completed at Biddison Run in Baltimore, MD |
Explaining
the genesis for his somewhat controversial—but now generally accepted—method of
stream classification, Rosgen says, “The requirement for more detailed,
reproducible, quantitative applications at various levels of inventory over wide
hydrophysiographic provinces has led to further development of classification
schemes.”
The
Rosgen System
Thus
was born a new system of stream classification that bears the name of its
creator. The “Rosgen Method” broadly involves four levels of
assessment:
I. Broad morphological
characterization (i.e., river profile morphology, general river pattern, basin
relief, and valley morphology)
II. Description of stream type (involving
channel patterns, entrenchment ratio, width/depth ratio, sinuosity, channel
material, and slope)
III. Stream condition (which includes, among other
elements, riparian vegetation, depositional patterns, fish habitat, flow regime,
channel stability, and bank erodibility)
IV. Field verification (requires direct
measurements and observations of sediment transport, bank erosion rates,
aggradation/degradation, hydraulic geometry, fish biomass, and riparian
vegetation)
Level
I relies heavily on aerial photography and/or topographic maps to differentiate
valley types into one of 11 forms and to classify a stream as one of eight broad
types.
Level
II, utilizing field measurements, involves much more specific stream
differentiation, categorizing a stream into one of 94 variations. Once a stream
has been accurately classified, educated projections may be made regarding its
sensitivity to disturbance, the streambank
erosion potential, the level of influence of riparian vegetation, and the
stream’s likely recovery potential.
Level
III analyzes the spatial and temporal variations in both the stream and its
watershed to determine the causes and extent of its instability. For this stage,
Rosgen describes what he refers to as a departure analysis, comparing the stream
being analyzed to a stable reference channel.
Level
IV is used to provide information on channel processes within specific stream
reaches. It is also used to evaluate prediction methodologies and the
effectiveness of repair and restoration efforts by stream
type.
One
might ask how all of the detailed information obtained from such stream
assessment is used. Rosgen wrote in an article titled “Restoration
WARSSS”:
“Changes
to the morphological, sedimentological, hydraulic, and biological character of
river channels must be compared to a stable reference stream representing the
same valley type. Thus, the nature, extent and consequence of departure must be
understood to relate observed characteristics to ‘potential’ states. If this
information was not collected and analyzed, then how would a river restoration
designer know how wide, deep, straight, crooked, steep, etc. to make the stream?
What should be the stable dimension, pattern, and profile? Can the restored
stream move the largest sediment size, can it move the sediment load? These are
not simple questions, nor are there simple solutions. However, methods are
available to make these assessments and calculations in order to reduce some of
this uncertainty in river restoration.”
In
fact, the four assessment levels above are part of Rosgen’s larger eight-phase
comprehensive restoration process, which describe the entire stream
repair/restore project from beginning to end. These sequential phases are
defined in the “Rosgen Geomorphic Channel Design” chapter of the NRCS
Stream Restoration Handbook:
- Define specific restoration objectives
associated with physical, biological, and/or chemical
processes.
- Develop regional and localized specific
information on geomorphologic characterization, hydrology, and
hydraulics.
- Conduct a watershed/river assessment to
determine river potential, current state, and the nature, magnitude, direction,
duration, and consequences of change. Review land use history and time trends of
river change. Isolate the primary causes of
instability and/or loss of physical and biological function. Collect and analyze
field data including reference reach data to define sedimentological, hydraulic,
and morphological parameters. Obtain concurrent biological data (limiting factor
analysis) on a parallel track with the physical data.
- Initially consider passive restoration
recommendations based on land use change in lieu of mechanical restoration. If
passive methods are reasonable to meet objectives, skip to the monitoring phase
(phase 8). If passive efforts and/or recovery potential do not meet stated
multiple objectives, proceed with the following phases.
- Initiate natural channel design with
subsequent analytical testing of hydraulic and sediment transport relations
(competence and capacity).
- Select and design
stabilization/enhancement/vegetative establishment measures and materials to
maintain dimension, pattern, and profile
to meet stated objectives.
- Implement the proposed design and
stabilization measures involving layout, water quality control, and construction
staging.
- Design a plan for effectiveness, validation, and
implementation monitoring to ensure stated objectives are met, prediction
methods are appropriate, and the construction is implemented as designed. Design
and implement a maintenance plan.
Not
Everyone Agrees
There
are those who take exception to Rosgen’s methodologies. Some of the criticism
includes:
- Rosgen’s Level I and Level II
analyses cannot be used to predict river behavior and are unreliable for
determining correct mitigation processes.
Rosgen’s
response: The Level I and Level II data are not intended to predict river behavior or to recommend specific restoration
approaches in and of themselves. The Level III and Level IV stages must also be
completed before any such assessments may be made.- Stream restoration should
avoid using form-based methods and instead use process-based
approaches.
Rosgen’s
response: Form and process are not mutually exclusive; they are critically
linked. River morphology (form) reflects boundary conditions and flow
processes. Variation in either boundary condition or flow process will alter
channel morphology. - Using the Rosgen
classification, channels have to be fit into some category, whether it is
appropriate or not.
Rosgen’s
response: The stream classification system was developed from morphological
measurements of hundreds of rivers, indicating a trend of variables and their
ranges that were then grouped into discreet channel types; they were not
“force-fed” into arbitrary units. A dual stream type is designated when selected
delineative variables overlap between types rather than “forcing” a
classification category.
Rosgen
can be quite blunt in his assessment of his critics. “The people who are most
adamantly opposed [to his stream classification system],” he says, “are those
who are the least familiar with it and who aren’t in the business of river
restoration. They don’t offer alternatives.”
He
is particularly irked by those who claim that his system is all about morphology
(form), while ignoring process (i.e., sediment transport, changes in flow, and
erosion). “Form is created from process,” he maintains. “Form and process are
directly integrated.” Indeed, many of his published papers are clear on this
point.
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Photo: Meadville Land Services |
| After Baltimore's Biddison Run stream was restored |
Regarding
restoration efforts, he is insistent on taking a comprehensive approach in which
“analytical, morphological, and empirical relationships are carefully analyzed
in order to determine a restoration approach.” His goal is to understand a
stream’s natural flow and to match mathematically the stable form of the stream
to channel reaches being repaired or restored.
Far
from sitting in an ivory tower of academic research, he stresses that he is
“putting principles into practice” gained from more than 40 years of working in
the field. After his 20 years with the US Forest Service, he says, “I wanted to
come up with an alternative” to the frequent use of riprap, concrete, and other
artificial methods of restoration. He much prefers using native
material.
In
his book, Applied
River Morphology,
Rosgen explains the ultimate goal of performing such careful and exacting
assessments:
“Natural
stream channel stability is achieved by allowing the river to develop a stable
dimension, pattern, and profile such that, over time, channel features are
maintained and the stream system neither aggrades nor degrades. For a stream to
be stable, it must be able to consistently transport its sediment load, both in
size and in type, associated with local deposition and scour. Channel
instability occurs when the scouring process leads to degradation, or excessive
sediment deposition results in aggradation. When the stream laterally
migrates,
but maintains its bankfull width and width/depth ratio, stability is achieved
even though the river is considered to be an active and dynamic
system.
 |
Photo: Skelly & Loy |
| Before restoration was completed at the First Hollow Run stream |
“The
consistency of dimension, pattern, and profile that exists among rivers is more
than chance or spurious correlation. Mathematical relations exist illustrating a
stratification of river systems by unique morphological forms that provide
meaning in an otherwise random appearing, complex set of interrelated variables.
Whenever proper attention to the ‘rules of the river’ is not respected, adverse
channel adjustments often result in damage to personal property and loss of
life.”
First
Hollow Run
“When
I was a kid, I used to jump over this stream,” the old-timers told Tracy Litwiler,
project manager for Meadville Land Service in Pennsylvania. But First Hollow
Run, a tributary of the Lehigh River in picturesque Carbon County, PA, had grown
to as much as 100 feet wide and up to 30 or 40 feet deep in spots. Litwiler
explains that the stream is located in a historical mining area. The stream that
had once been narrow and shallow had suffered severe
scour.
In
2001, Pennsylvania Growing Greener grants were issued to begin the repair
process and the Harrisburg, PA, engineering firm Skelly & Loy was hired. The
company’s vice president of environmental engineering, Gerald Longenecker,
describes what he found.
“First
Hollow Run is located in a very steep area, with an approximate 11% slope, and
the coal mining resulted in this stream being diverted around a mine spoil,
together with the neighboring stream, Second Hollow Run. The contents of the
mine spoil were nothing like compacted soil, so water started chewing away at
the edges of the mine spoil, which was composed of easily eroded
material.”
 |
Photo: Skelly & Loy |
| After restoration was completed at the First Hollow Run stream |
Because
the channel was very steep, and because Second Hollow Run had effectively merged
with First Hollow Run, there was a massive amount of energy in the water flow.
As a result, the combined stream rapidly eroded a deep, V-shaped ravine to the
point that a formerly buried water pipe at the stream’s edge had been exposed
with the local water authorities fearful of a pipe
failure.
“This
stream should have been 20 feet wide and two or three feet deep,” says
Longenecker, “but the banks and sidewalls were being washed into the stream
corridor, and both sides of the channel were being eroded to the point that even
areas well away from the mine spoil were being damaged.”
In
describing his initial stream assessment approach, Longenecker explains, “The
concepts of fluvial geomorphology that Rosgen preaches were heavily used in
establishing cross-sectional dimensions of the main channel [‘bankfull channel’
in his terminology] and also minimum flood plain width to locate the base of the
rock wall [at the edge of the stream]. Other techniques were blended with his
methods to design the boulder step structures”
that were used in reducing flow velocity.
In
the vicinity of the old mining operations were fields of massive boulders, 10
feet or more in width, that were essentially waste material, as Longenecker
recalls. “But this negative became a positive in the restoration process,” he
says. “This was exactly what we needed—huge boulders or mass to resist the
intense energy of the water. And this was used as the first defense to restore
stability to the stream. The landowner gave us permission to use these boulders
and this saved hundreds of thousands of dollars on the project. They were all
within about a mile of the site.” More than 3,000 tons of boulder rock was used.
Engineers at Skelly & Loy later presented a technical paper describing their
creation of such a step-pool stream channel (Rosgen A-type stream) using large
“keystone” boulders to produce a natural-looking channel.
Project
manager Litwiler points out another negative-turned-positive. There were a large
number of trees located on the upper banks of the stream, but the eroding soil
could no longer support the root structure. Trees were being uprooted and
falling into the river.
With
trees that had already toppled, or that were in danger of doing so, Litwiler
made use of the large mass of material. “We separated the root mass and the
first 10 to 15 feet of log from the rest of the tree and turned it into a root
wad revetment. This root wad was placed in the water and oriented so as to
oppose the stream flow. The root wad was anchored into the slope with rocks and
more logs, and the result was that this manipulated the base course flow of the
stream. It also created a habitat for fish. It’s much less costly when onsite
materials are available to be used in this manner.”
Furthermore,
he says, crews then cut the rest of the tree into pieces and used the pieces as
“bunny huts” to provide shelter to small animals in the area. As a result, every
portion of these failed or failing trees was used to improve the environment in
and around the stream.
Longenecker
recalls a surprising turn of events at the site. “A spoil pile on one side of
the channel was actually higher than the tops of the houses on the other side of
the channel.” The plan was to substantially grade back this spoil, but it turned
out that the homeowners in the area actually preferred that the massive spoil
remain in place, because it provided a nice windbreak for
them.
“Instead,”
says Longenecker, “we planted many trees and brush to build a natural fence or
filter that runs parallel to the stream about 20 to 40 feet away from the
water’s edge, on the same side as the spoil mound. This acts to catch material
coming down the spoil bank. So although there is still a very steep bank where
the mine spoil is located, we have managed to arrest erosion at the base of the
slope and the area is slowly stabilizing.”
Another
part of the restoration effort was to create a series of mini-waterfalls
cascading into a deep pool area that was created by another large boulder about
20 feet down the stream channel. Each step along this cascading waterfall
dissipated some of the energy of the water, decreasing turbulence. Not only did
this help to stem the erosion, but a couple of the homeowners even built small
observation areas where they could sit and enjoy the sound of the rushing water,
Longenecker says.
There
were indeed many challenges encountered
and unique solutions instituted. By the time the restoration was complete in
2006, 1,300 linear feet of First Hollow Run had been reconstructed, reducing its
width to 19 feet, with a depth of some 2.5 feet. The project was such a success
that engineering firm Skelly & Loy was awarded a Diamond Award Certificate
for Engineering Excellence by the American Council of Engineering Companies of
Pennsylvania.
Three
Forks Ranch Stream Restoration
Straddling
the Colorado-Wyoming border, the
massive Three Forks Ranch lies along the Little Snake River. It is claimed that
the restoration work undertaken on this portion of the Little Snake River
represents the largest privately funded river restoration project in US
history.
Ranch
general manager Jay Linderman wrote about the huge effort in the fall 2001
newsletter of the Colorado Riparian Association. He explains, “The river bottom,
being fertile, easily accessible and irrigable, was overused and abused for many
years. Overgrazing and haying resulted in the destruction of the willow
community along the riverbanks. Bank degradation soon followed and a
near-irreversible cycle began. Over time, the river channel became wide and
shallow. Stream banks became abrupt drops of 3 to 6 feet into the river. Each
spring, snowmelt runoff added to the problem. Water temperatures increased and
dissolved oxygen levels declined. These two factors, along with increased
sedimentation, greatly reduced the fish habitat in the
river.”
The
ranch brought in Dave Rosgen to assess the situation and to implement the river
reconstruction. “The restoration goals,” Linderman wrote, “were to restore
natural stream bank stability, establish woody riparian vegetation, greatly
reduce bank erosion, create wetlands, and improve fish habitat. This required
deepening and narrowing the river channel, which would lead to cooler water
temperatures and increased dissolved oxygen levels. The steep, eroded banks were
removed and native cobble was used as a revetment in an effort to eliminate
future erosion.”
In
addition, “Approximately 22,000 large boulders were placed along about four
miles of river in the form of Rosgen-designed structures [e.g., J-hooks,
W-wiers, and cross-vanes] to divert the stream’s energy away from the banks and
toward the center of the stream. The rock structures also created pool for fish.
In reaches of the river that had been straightened in the past to facilitate
irrigation or road building, the natural sinuosity of the channel was restored.
When the massive reconstruction was completed, mature willows and sod were
transplanted on the outside of every meander bend to further stabilize the banks
and to create shade to reduce water temperature.”
The
unprecedented restoration effort was such that downstream neighbors of the ranch
requested that their portions of the river also be reconstructed to improve
function and stability. To date, some 16 miles of the Little Snake River have
now been restored to what it may have looked like a century or two ago.