Dammed rivers are subject to changes in their flow, water-quality, and sediment regimes. Each of these changes may contribute to diminished aquatic habitat quality and quantity. Of the three factors, an altered sediment regime is a particularly unyielding challenge on many dammed rivers. The magnitude of the challenge is illustrated on the Lower Missouri River, where the largest water storage system in North America has decreased the downriver suspended-sediment load to 0.2%–17% of pre-dam loads. In response to the altered sediment regime, the Lower Missouri River channel has incised as much as 3.5 m just downstream of Gavins Point Dam, although the bed has been stable to slightly aggrading at other locations farther downstream. Effects of channel engineering and commercial dredging are superimposed on the broad-scale adjustments to the altered sediment regime. The altered sediment regime and geomorphic adjustments constrain restoration and management opportunities. Incision and aggradation limit some objectives of flow-regime management: In incising river segments, ecologically desirable reconnection of the floodplain requires discharges that are beyond operational limits, whereas in aggrading river segments, small spring pulses may inundate or saturate low-lying farmlands. Lack of sediment in the incising river segment downstream of Gavins Point Dam also limits sustainable restoration of sand-bar habitat for bird species listed under the Endangered Species Act. Creation of new shallow-water habitat for native fishes involves taking sediment out of floodplain storage and reintroducing most or all of it to the river, raising concerns about increased sediment, nutrient, and contaminant loads. Calculations indicate that effects of individual restoration projects are small relative to background loads, but cumulative effects may depend on sequence and locations of projects. An understanding of current and historical sediment fluxes, and how they vary along the river, provides a quantitative basis for defining management constraints and identifying opportunities.

Conversion of upland forest and prairie vegetation to agricultural land uses, following Euro-American settlement in the Upper Mississippi River System, led to accelerated runoff and soil erosion that subsequently transformed channels, floodplains, and wetlands on bottomlands. Halfway Creek Marsh, at the junction of Halfway Creek and the Mississippi River on Wisconsin’s western border, is representative of such historical transformation. This marsh became the focus of a 2005–2006 investigation by scientists from the U.S. Geological Survey, the University of Wisconsin–Madison, and the U.S. Environmental Protection Agency, who used an understanding of the historical transformation to help managers identify possible restoration alternatives for Halfway Creek Marsh. Field-scale topographic surveys and sediment cores provided data for reconstructing patterns and rates of historical overbank sedimentation in the marsh. Information culled from historical maps, aerial photographs, General Land Office Survey notes, and other historical documents helped establish the timing of anthropogenic disturbances and document changes in channel patterns. Major human disturbances, in addition to agricultural land uses, included railroad and road building, construction of artificial levees, drainage alterations, and repeated dam failures associated with large floods. A volume of approximately 1,400,000 m 3 , involving up to 2 m of sandy historical overbank deposition, is stored through the upper and lower marshes and along the adjacent margins of Halfway Creek and its principal tributary, Sand Lake Coulee. The estimated overbank sedimentation rate for the entire marsh is ~3,000 m 3 yr ‒1 for the recent period 1994–2006. In spite of reduced surface runoff and soil erosion in recent years, this recent sedimentation rate still exceeds by ~4 times the early settlement (1846–1885) rate of 700 m 3 yr ‒1 , when anthropogenic acceleration of upland surface runoff and soil erosion was beginning. The highest rate of historical bottomland sedimentation occurred from 1919 to 1936, when the estimated overbank sedimentation rate was 20,400 m 3 yr ‒1 . This rate exceeded by nearly 30 times the 1846–1886 rate. Artificial levees were constructed along the upper reach of Halfway Creek in the marsh during the early twentieth century to restrict flooding on the adjacent bottomlands. Anomalously high overbank sedimentation rates subsequently occurred on the floodplain between the levees, which also facilitated more efficient transport of sediment into the lower marsh bottomland. Although overbank sedimentation rates dropped after 1936, corresponding to the widespread adoption of soil-conservation and agricultural best-management practices, the continuation of anomalously high overbank sedimentation between the levees led to increased bank heights and development of a relatively deep channel. The deep cross-section morphology is commonly mistaken as evidence of channel incision; however, this morphology actually resulted from excessive overbank sedimentation. The historical metamorphosis of the Halfway Creek channel and riparian wetlands underscores the importance of understanding the long-term history of channel and floodplain evolution when restoration of channels and riparian wetlands are under consideration. Sedimentation patterns and channel morphology for Halfway Creek Marsh probably are representative of other anthropogenically altered riparian wetlands in the Upper Mississippi River System and similar landscapes elsewhere.

Regional high-magnitude rainstorms have produced several large floods in north coastal California during the last century, which resulted in extensive mass-movement activity and channel aggradation. Channel monitoring in Redwood Creek, through the use of cross-sectional surveys, thalweg profiles, and pebble counts, has documented the persistence and routing of channel-stored sediment following these large floods in the 1960s and 1970s. Channel response varied on the basis of timing of peak aggradation. Channel-stored sediment was evacuated rapidly from the upstream third of the Redwood Creek channel, and the channel bed stabilized by 1985 as the bed coarsened. Currently only narrow remnants of flood deposits remain and are well vegetated. In the downstream reach, channel aggradation peaked in the 1990s, and the channel is still incising. Channel-bed elevations throughout the watershed showed an approximate exponential decrease with time, but decay rates were highest in areas with the thickest flood deposits. Pool frequencies and depths generally increased from 1977 to 1995, as did median residual water depths, but a 10 yr flood in 1997 resulted in a moderate reversal of this trend. Channel aggradation generated during 25 yr return interval floods has persisted in Redwood Creek for more than 30 yr and has impacted many life cycles of salmon. Watershed restoration work is currently focused on correcting erosion problems on hillslopes to reduce future sediment supply to Redwood Creek instead of attempting in-channel manipulations.

Hydraulic gold-mining tailings produced in the late nineteenth century in the Sierra Nevada foothills of California caused severe channel aggradation in the lower Feather and Yuba Rivers. Topographic and planimetric data from historical accounts, maps, topographic surveys, vertical sections, aerial photographs, and LiDAR (light detection and ranging) data reveal contrasting styles of channel change and floodplain evolution between these two rivers. For example, levee cross-channel spacings up to 4 km along the lower Yuba River contrast with spacings <2 km on the larger Feather River. More than a quarter billion cubic meters of hydraulic-mining sediment were stored along the lower Yuba River, and the wide levee spacing was intentionally maintained during design of the flood-control system to minimize delivery of sediment to navigable waters downstream. Consequently, the lower Yuba floodplain has a multithread high-water channel system with braiding indices >12 in some reaches. Some of the larger of these channels remain clearly visible on aerial photographs and LiDAR imagery in spite of intensive agricultural leveling. Narrow levee spacings on the Feather River were designed to encourage transport of mining sediment downstream and keep the channel clear for navigation. Levee spacings on the lower Feather River reached a minimum near the turn of the twentieth century, when floodplain widths were reduced at several constricted reaches to <250 m. Historical data indicate that the general channel location of the lower Yuba River had stabilized by the end of the nineteenth century, whereas substantial channel avulsions began later and continued into the twentieth century on the lower Feather River. The striking contrasts in channel change between the Yuba and Feather Rivers are due, at least in part, to different river-management strategies, although the Yuba River received much more sediment. Early river engineering of these channels represented the first efforts at integrated river-basin management west of the Mississippi, so the observed long-term effects are instructive. Modern river management should consider how the disturbance factors in these channels and the imprint of early river management affect the modern morphologic stability and sediment-production potential of the channel and floodplain.

This paper deals with channel evolution over the past 200 yr in 12 selected streams in northern and central Italy and aims at reconstructing the evolutionary trends (e.g., trends of channel width and bed elevation) and understanding the causes of channel adjustments. The selected streams have been studied using various sources and methods (historical maps, aerial photographs, topographic surveys, and geomorphological surveys). The selected rivers have undergone almost the same processes in terms of temporal trends; however, the magnitude of adjustments varies according to several factors, such as original channel morphology. Initially, river channels underwent a long phase of narrowing (up to 80%) and incision (up to 8–10 m), which started at the end of the nineteenth century and was intense from the 1950s to the 1980s. Then, over the last 15–20 yr, channel widening and sedimentation, or bed-level stabilization, have become the dominant processes in most of the rivers. Different human interventions have been identified as the causes of channel adjustments in Italian rivers (sediment mining, channelization, dams, reforestation, and torrent control works). Such interventions have caused a dramatic alteration of the sediment regime, whereas effects on channel-forming discharges have seldom been observed. Some notable implications for river management and restoration are (1) the state of rivers before major human disturbances and channel adjustments can rarely be taken as a reference, as at present rivers are far from their pristine condition; and (2) sediment management is and will be a key issue in such fluvial systems.

Dam construction and its impact on downstream fluvial processes may substantially alter ambient bank stability and erosion. Three high dams (completed between 1953 and 1963) were built along the Piedmont portion of the Roanoke River, North Carolina; just downstream the lower part of the river flows across largely unconsolidated Coastal Plain deposits. To document bank erosion rates along the lower Roanoke River, >700 bank-erosion pins were installed along 66 bank transects. Additionally, discrete measurements of channel bathymetry, turbidity, and presence or absence of mass wasting were documented along the entire study reach (153 km). A bank-erosionfloodplain-deposition sediment budget was estimated for the lower river. Bank toe erosion related to consistently high low-flow stages may play a large role in increased mid- and upper-bank erosion. Present bank-erosion rates are relatively high and are greatest along the middle reaches (mean 63 mm/yr) and on lower parts of the bank on all reaches. Erosion rates were likely higher along upstream reaches than present erosion rates, such that erosion-rate maxima have since migrated downstream. Mass wasting and turbidity also peak along the middle reaches; floodplain sedimentation systematically increases downstream in the study reach. The lower Roanoke River is net depositional (on floodplain) with a surplus of ~2,800,000 m 3 /yr. Results suggest that unmeasured erosion, particularly mass wasting, may partly explain this surplus and should be part of sediment budgets downstream of dams.

Effective watershed-scale environmental management and restoration require a sound understanding of the dynamics of fluvial systems at the watershed-scale and the impact of humans on these dynamics. In Illinois, concern has arisen about the need to implement bank stabilization along meandering rivers, where bank erosion associated with lateral migration is often viewed as a sign of channel instability. Also, many rivers in the state are low-energy fluvial systems that exhibit limited responses to direct human modification such as channel straightening. From an ecological perspective, the lack of response is problematic owing to its potential long-term alteration of aquatic habitat. This study examines the spatial relationship between the planform dynamics of meandering rivers and stream power in the Kishwaukee River watershed in northern Illinois. The spatial extent of planform change at the scale of drainage network is quantified and related to spatial variations in the magnitude of stream power throughout the watershed. Historical channel change was determined using GIS-based analysis of aerial photography of several reaches scattered throughout the watershed. The results show that the amount of lateral migration per reach is greatest where stream power is highest, but that planform response to channelization is limited regardless of the magnitude of stream power. The findings from this historical analysis of channel change are useful for understanding both the fluvial dynamics of unmodified meandering rivers and the influence of human modification on these dynamics— knowledge that can help guide environmental decision making about the need to implement channel stabilization or restoration measures.

There is clear evidence that the Minnesota River is the major sediment source for Lake Pepin and that the Le Sueur River is a major source to the Minnesota River. Turbidity levels are high enough to require management actions. We take advantage of the well-constrained Holocene history of the Le Sueur basin and use a combination of remote sensing, field, and stream gauge observations to constrain the contributions of different sediment sources to the Le Sueur River. Understanding the type, location, and magnitude of sediment sources is essential for unraveling the Holocene development of the basin as well as for guiding management decisions about investments to reduce sediment loads. Rapid base-level fall at the outlet of the Le Sueur River 11,500 yr B.P. triggered up to 70 m of channel incision at the mouth. Slope-area analyses of river longitudinal profiles show that knickpoints have migrated 30–35 km upstream on all three major branches of the river, eroding 1.2–2.6 × 10 9 Mg of sediment from the lower valleys in the process. The knick zones separate the basin into an upper watershed, receiving sediment primarily from uplands and streambanks, and a lower, incised zone, which receives additional sediment from high bluffs and ravines. Stream gauges installed above and below knick zones show dramatic increases in sediment loading above that expected from increases in drainage area, indicating substantial inputs from bluffs and ravines.

The South Platte and Republican River basins provide examples of historical channel changes on the western Great Plains. Flow regulation and diversion caused substantial channel narrowing and vegetation encroachment along larger, perennial rivers that head in the Rocky Mountains. Intensive groundwater pumping has reduced the volume and longitudinal connectivity of refuge pools along smaller intermittent or ephemeral channels that head on the plains. A case study from the Pawnee National Grassland of Colorado illustrates the dynamics of intermittent streams, as well as measures that can be taken to protect and restore refuge pools along these streams. The implications of channel change, and the need to protect and rehabilitate rivers, are less widely recognized for smaller rivers of the western Great Plains than for the larger, perennial rivers. Our objectives in this chapter are to provide a regional context for understanding changes in smaller plains rivers during the past century by reviewing the diversity of channel types and historical changes in the western Great Plains, and to briefly explore the dynamics of smaller plains rivers and the challenges to preserving these riverscapes.

Streamflow on the North Fork Cache La Poudre River, a tributary of the South Platte River in north-central Colorado, has been modified by impoundments for a century. A proposed expansion of the largest reservoir on the North Fork, Halligan Reservoir, presents an opportunity to modify dam operation to achieve environmental flows that sustain the river ecosystem while augmenting municipal water supplies. Over the past century, decreases in flood-related disturbances have resulted in reduced bed scouring through sediment transport and significant shifts in community composition and population structure of the dominant woody species present along the North Fork. We propose a four-step method to characterize environmental flows that maintain sediment mobility and riparian vegetation composition and structure. Our environmental flow standards explicitly address the fundamental role of sediment in creating and maintaining riparian habitat. Environmental flows to transport bedload are lower magnitude, higher frequency events (2 yr recurrence interval) that serve many in-channel functions, whereas environmental flows directed at riparian vegetation respond over longer time scales to high magnitude, lower frequency events (10 yr and 25 yr floods). Field evidence suggests the need for a 10 yr flood to saturate over-bank areas and exclude xeric species near the channel. Managed 25 yr floods serve as a target flow for generating canopy gaps, creating regenerative habitat, enhancing biogeochemical processes, maintaining habitat heterogeneity, and possibly disrupting the coarse bed-surface layer and scouring pools to maintain fish overwinter habitat within the North Fork. Where field evidence is lacking, selection of target flows can be guided by daily discharge exceedence values.

River discharge is the fundamental process that operates in a fluvial system. The increase in discharge and drainage area downstream is intuitive, but data sets that describe this increase within individual watersheds are not common. The scaling of discharge and drainage area can be described as Q = kA c , where Q is river discharge, A is drainage area, and k and c are scaling constants. The variable k is not often illustrative of watershed processes, but the constant c represents the rate at which discharge ( Q ) increases downstream when compared to drainage area ( A ). This study compiles the annual peak discharge records of rivers from U.S. Geological Survey (USGS) gauges to determine the rate ( c ) at which discharge and drainage increase downstream. The peak annual discharge records were selected to represent a variety of watersheds spanning multiple climatic and geographic settings as well as to illustrate the effects of anthropogenic land-use change and river-management practices over the length of the records. It is often assumed that the scaling between discharge and drainage area is linear ( c ~1), and 16 of these rivers exhibit this behavior over the length of their record. However, most of the rivers studied show nonlinear behavior and/or secular trends in their c values. Eleven rivers have peak annual discharge scaling values ( c ) of <1, three have c values substantially larger than 1, and ten exhibit secular changes in c over part or all of their records. These nonlinear and changing c values can be attributed to both natural and anthropogenic causes, such as dams, urbanization, and other land-use changes. These c values indicate the need for caution before assuming that discharge and drainage area are linearly related.

Geomorphic changes in reconfigured reaches of three Colorado rivers in response to floods in 2005 provide a benchmark for “restoration” assessment. Sediment-entrainment potential is expressed as the ratio of the shear stress from the 2 yr, 5 yr, 10 yr, and 2005 floods to the critical shear stress for sediment. Some observed response was explained by the excess of flood shear stress relative to the resisting force of the sediment. Bed-load entrainment in the Uncompahgre River and the North Fork Gunnison River, during 4 and 6 yr floods respectively, resulted in streambed scour, streambed deposition, lateral-bar accretion, and channel migration at various locations. Some constructed boulder and log structures failed because of high rates of bank erosion or bed-material deposition. The Lake Fork showed little or no net change after the 2005 flood; however, this channel had not conveyed floods greater than the 2.5 yr flood since reconfiguration. Channel slope and the 2 yr flood, a surrogate for bankfull discharge, from all three reconfigured reaches plotted above the Leopold and Wolman channel-pattern threshold in the “braided channel” region, indicating that braiding, rather than a single-thread meandering channel, and midchannel bar formation may be the natural tendency of these gravel-bed reaches. When plotted against a total stream-power and median-sediment-size threshold for the 2 yr flood, however, the Lake Fork plotted in the “single-thread channel” region, the North Fork Gunnison plotted in the “multiple-thread” region, and the Uncompahgre River plotted on the threshold. All three rivers plotted in the multiple-thread region for floods of 5 yr recurrence or greater.

Although step-pools are increasingly used in stream restoration to stabilize steep channels, few studies have examined artificially manipulated step-pool systems after restoration. Whereas monitoring efforts have emphasized morphological change within restored systems, knowledge of the ecological potential for restoration using step-pool sequences is particularly incomplete. Baxter Creek (El Cerrito, Contra Costa County) and Codornices Creek (Berkeley, Alameda County) in California provide two unique cases of restored step-pool systems with which to assess post-restoration responses. Since restoration was completed in 1996 and 2003, respectively, Baxter Creek and Codornices Creek have achieved geomorphic stability, characterized by maximum flow resistance and minor change in channel cross sections. The integrity of the restored step-pool channels has been maintained through storms that have exceeded the 14–20+ year recurrence interval. Comparison of the types and characteristics of benthic macroinvertebrates in the restored reaches with unrestored sites upstream and downstream, as well as with a reference channel (Wildcat Creek, Contra Costa County), indicates that restoration has effectively created ecological environments consistent with the overall watershed settings within Baxter Creek and Codornices Creek. Biological metrics used to represent ecological conditions indicate that Wildcat Creek, the reference site, had a healthier condition, which was reflected in a higher percentage of sensitive taxa (e.g., Ephemeroptera, Plecoptera, Trichoptera or EPT) and a lower percentage of tolerant taxa (e.g., oligochaetes and midge larvae). No significant differences were found between the restored reaches in Baxter Creek and Codornices Creek, and those upstream and downstream of the restored sites. Comparison of the biological metrics among habitat types (steps, pools, riffles) within each stream and across watersheds indicates a tendency toward higher biological quality (characterized by a higher percentage of sensitive organisms) in steps compared with pools. These findings suggest the ecological potential of stream restoration using step-pools, because step clasts may offer habitats for higher percentages of sensitive and specialized organisms.

West Tennessee has a complex history of watershed disturbance, including agricultural erosion, channelization, accelerated valley sedimentation, and the removal and reestablishment of beaver. Watershed management has evolved from floodplain drainage via pervasive channelization to include local drainage canal maintenance and local river restoration. Many unmaintained canals are undergoing excessive aggradation and complex channel evolution driven by upland erosion and low valley gradient. The locus of aggradation in fully occluded canals (valley plugs) moves up-valley as sediment continues to accumulate in the backwater behind the plug. Valley plugs that cause canal avulsion can lead to redevelopment of meandering channels in less disturbed areas of the floodplain, in a process of passive self-restoration. Some valley plugs have brought restored floodplain function, reoccupation of extant historic river channels, and formation of a “sediment shadow” that protects downstream reaches from excess sedimentation. Despite the presence of numerous opportunities, there is presently no mechanism for including valley plugs in mitigation projects. In 1997 a survey of 14 reference reach cross sections documented relations between drainage area and bankfull geometry of relatively unmodified streams in West Tennessee. Reassessment of seven of those sites in 2007 showed that one had been dammed by beaver and that two sites could not be analyzed further because of significant vertical or lateral instability. In contrast to other regions of North America, the results suggest that stream channels in this region flood more frequently than once each year, and can remain out of banks for several weeks each year.

No Abstract Available.

No Abstract Available.