Cliff R. Hupp

Riparian vegetation and fluvial geomorphic processes

Abstract Riparian vegetation and fluvial-geomorphic processes and landforms are intimately connected parts of the bottomland landscape. Relations among vegetation, processes, and landforms are described here for representative streams of four areas of the United States: high-gradient streams of the humid east, coastal-plain streams, Great Plains streams, and stream channels of the southwestern United States. Vegetation patterns suggest that species distributions in the humid east are largely controlled by frequency, duration, and intensity of floods. Along channelized streams, vegetation distribution is largely controlled by variation in fluvial geomorphic processes (cycles of degradation and aggradation) in response to increases in channel gradient associated with channelization. Similarly, riparian vegetation of Great Plains streams may be controlled by fluxes in sediment deposition and erosion along braided streams. Patterns of riparian vegetation in semi-arid regions may be most closely related to patterns of water availability, unlike most other streams in more humid environments. Channel-equilibrium conditions control stability of the coincident fluvial landform and attendant vegetation pattern throughout the continent. In most situations, riparian-vegetation patterns are indicative of specific landforms and, thus, of ambient hydrogeomorphic conditions.

Hydrological and geomorphological impacts on riparian plant communities.

Riparian vegetation is affected by both flood processes and the characteristics of landforms that are shaped by floods. In many instances, species occurrence can be linked directly to specific fluvial landforms. These spatial relationships are largely due to the role of floods in the differential destruction of vegetation, in the determination of substrate characteristics, and in the transport of propagules. Major floods may, depending on the climatic context, allow for the establishment of stands of vegetation, or restart processes of plant community change. Disturbance by floods can also affect biodiversity: species richness in some watersheds is greatest where steep valley floor gradients allow for high-energy floods. The recognition and analysis of hydrogeomorphological influences on riparian vegetation are complicated by multiple scales of environmental interactions, by the covariance of some environmental variables, and by feedbacks between vegetation and flood regimes. Copyright

Bottomland vegetation distribution along Passage Creek, Virginia, in relation to fluvial landforms.

Persistent distribution patterns of woody vegetation within the bottomland forest of Passage Creek, Virginia, were related to fluvial landforms, channel geometry, streamflow characteristics, and sediment—size characteristics. Vegetation patterns were determined from species presence as observed in transects and traverses on landforms developed along the stream. Distinct species distributional patterns were found on four common fluvial geomorphic landforms: depositional bar, active—channel shelf, floodplain, and terrace. Independent hydrologic characteristics (flow duration and flood frequency) were determined for each of the landforms. Vegetation data were analyzed by binary discriminant analysis, principal components analysis, and detrended correspondence analysis. Results and related field observations suggest that certain species are significantly associated with specific fluvial landforms. Vegetation patterns appear to develop more as a result of hydrologic processes associated with each fluvial landform rather than from sediment—size characteristics. Flood disturbance may be an important factor in maintaining the vegetation patterns, which may therefore be used as indicators for particular hydrogeomorphic site conditions.

Riparian Vegetation Recovery Patterns Following Stream Channelization: A Geomorphic Perspective

Hundreds of kilometres of West Tennessee streams have been channelized since the turn of the century. After a stream is straightened, dredged, or cleared, basin- wide ecologic, hydrologic, and geomorphic processes bring about an integrated, character- istic recovery sequence. The rapid pace of channel responses to channelization provides an opportunity to document and interpret vegetation recovery patterns relative to otherwise long-term, concomitant evolution of river geomorphology. Nearly 150 sites along 15 streams were studied in the Obion, Forked Deer, Hatchie, and Wolf River basins. Channels of these streams, except that of the Hatchie River main stem, have undergone major modi- fications along all or parts of their courses. This paper presents the eco-geomorphic analyses and interpretation of a large multidisciplinary study, with special reference to the inter- related hydrogeomorphic aspects of channel recovery. Quantitative plant ecological anal- yses were conducted to infer relative bank stability, to identify indicator recovery species, and to determine patterns of vegetation development through the course of accelerated channel evolution. Binary-discriminant and ordination analyses show that distinctive ri- parian-species patterns reflect a six-stage model of channel evolution and can be used to infer channel stability and hydrogeomorphic conditions. Woody vegetation initially estab- lishes on low- and mid-bank surfaces at the same location and time that bank accretion begins, and corresponds to the site of initial geomorphic restabilization. The linkage of channel bed aggradation, woody vegetation establishment, and bank accretion all lead to recovery of the channel. Pioneer species are hardy and fast growing, and can tolerate moderate amounts of slope instability and sediment deposition; these species include river birch (Betula nigra), black willow (Salix nigra), boxelder (Acer negundo), and silver maple (Acer saccharinum). High stem densities and root-mass development appear to enhance bank stability. Tree-ring analyses suggest that on average 6 5 yr may be required for recovery after channelization.

Hydrology, geomorphology and vegetation of Coastal Plain rivers in the south-eastern USA.

Rivers of the coastal plain of the south-eastern USA are characteristically low-gradient meandering systems that develop broad floodplains subjected to frequent and prolonged flooding. These floodplains support a relatively unique forested wetland (bottomland hardwoods), which have received considerable ecological study, but distinctly less hydrogeomorphological study. The hydroperiod, or annual period of inundation, largely controls the development of characteristic fluvial landforms, sediment deposition and vegetation distribution patterns. Order-of-magnitude differences in wetted perimeter, width/depth, suspended sediment load and hydraulic roughness may exist between ‘dry’ in-channel seasons and the hydroperiod. Substantial sediment (and adsorbed contaminants) retention and storage through lateral and vertical accretion is common (where not heavily impacted by flow regulation) along these Coastal Plain rivers. The present chapter summarizes our current understanding of the hydrology, fluvial geomorphology, general and local sedimentation patterns, and related plant ecological patterns of these Coastal Pplain bottomlands.

CARBON, NITROGEN, AND PHOSPHORUS ACCUMULATION IN FLOODPLAINS OF ATLANTIC COASTAL PLAIN RIVERS, USA

Net nutrient accumulation rates were measured in riverine floodplains of the Atlantic Coastal Plain in Virginia, Maryland, and Delaware, USA. The floodplains were located in watersheds with different land use and included two sites on the Chickahominy River (urban), one site on the Mattaponi River (forested), and five sites on the Pocomoke River (agricultural). The Pocomoke River floodplains lie along reaches with natural hy- drogeomorphology and on reaches with restricted flooding due to channelization and levees. A network of feldspar clay marker horizons was placed on the sediment surface of each floodplain site 3-6 years prior to sampling. Sediment cores were collected from the material deposited over the feldspar clay pads. This overlying sediment was separated from the clay layer and then dried, weighed, and analyzed for its total carbon (C), nitrogen (N), and phosphorus (P) content. Mean C accumulation rates ranged from 61 to 212 g·m 22 ·yr 21 , N accumulation rates ranged from 3.5 to 13.4 g·m 22 ·yr 21 , and P accumulation rates ranged from 0.2 to 4.1 g·m 22 ·yr 21 among the eight floodplains. Patterns of intersite variation in mineral sediment and P accumulation rates were similar to each other, as was variation in organic sediment and C and N accumulation rates. The greatest sediment and C, N, and P accumulation rates were observed on Chickahominy River floodplains downstream from the growing metro- politan area of Richmond, Virginia. Nutrient accumulation rates were lowest on Pocomoke River floodplains that have been hydraulically disconnected from the main channel by channelization and levees. Sediment P concentrations and P accumulation rates were much greater on the hydraulically connected floodplain immediately downstream of the limit of channelization and dense chicken agriculture of the upper Pocomoke River watershed. These findings indicate that (1) watershed land use has a large effect on sediment and nutrient retention in floodplains, and (2) limiting the hydraulic connectivity between river channels

Geomorphic and vegetative characteristics along three northern Virginia streams.

Geometry, sediment, and woody-vegetation data were collected from bottomland geomorphic surfaces at valley sections along three gaged perennial streams of northern Virginia. The basins of the streams differ widely in topography and physiography; mean discharges vary from 0.196 to 323 m3 per sec. Prevalent surfaces identified were the depositional bar, the active-channel shelf, the flood plain, and terraces. The stages corresponding to active-channel-shelf levels were equivalent to flow durations of 5% to 13%. Stages corresponding to flood-plain levels were equivalent to discharges with 1.4- to 2.0-yr recurrence intervals. The discharge data and statistical tests of geomorphicsurface, sediment, and vegetative data suggest that the various alluvial features are formed and maintained by hydraulically controlled sorting processes. Analysis of woody-plant and geomorphic data shows that each surface supports characteristic species, some of which are nearly unique to a surface. Tests of sediment type with species distribution showed lower correlation than that of geomorphic surface with species. It is inferred that plant distributions largely are controlled by flow frequency and intensity, and that plants may help to identify geomorphic levels and potential for flood damage.

Bank accretion and the development of vegetated depositional surfaces along modified alluvial channels

This paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized West Tennessee streams. Most perennial streams in West Tennessee were straightened and dredged since the turn of the century. Patterns of fluvial ecological responses to channelization have previously been described by a six-stage model. Dendrogeomorphic (tree-ring) techniques allowed the determination of location, timing, amount, and rate of bank-sediment deposition. Channel cross sections and ecological analyses made at 101 locations along 12 streams, encompassing bends and straight reaches, show that channel and bank processes initially react vertically to channelization through downcutting. A depositional surface forms on banks once bed-degradation and heightened bank mass wasting processes have eased or slowed. The formation of this depositional surface marks the beginning of bank recovery from channelization. Dominating lateral processes, characteristic of stable or natural channels, return during the formation and expansion of the depositional surface, suggesting a relation with thalweg deflection, point-bar development, and meanderloop extension. Characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks. The depositional surface initially forms low on the bank and tends to maintain a slope of about 24°. Mean accretion rates ranges from 5.9 cm/yr on inside bends to 0 cm/yr on most outside bends; straight reaches have a mean-accretion rate of 4.2 cm/yr. The relatively stable, convex upward, depositional surface expands and ultimately attaches to the flood plain. The time required for the recovery process to reach equilibrium averaged about 50 years. Indicative pioneer speccies of woody riparian vegetation include black willow, river birch, silver maple, and boxelder. Stem densities generally decrease with time after and initial flush of about 160 stems per 100 m2. Together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation.

Temporal and spatial patterns of wetland sedimentation, West Tennessee

Dendrogeomorphic techniques were used to describe and interpret patterns of sedimentation rates at two forested wetland sites in West Tennessee. Fifty-five sampling stations were established along transects upstream and downstream from bridge structures, and 515 trees were examined for depth of sediment accretion and cored for age determination. Temporal variation in sedimentation rate may be related more to stream channelization and agricultural activity than to bridge and causeway construction. Sedimentation rates have increased substantially in the last 28 years, although channelized streams may have overall lower rates than unchannelized streams. Comparisons of sedimentation rates from deposition over artificial markers (short term) with those determined from tree-ring analysis (long-term) indicate that trends are similar where hydrogeomorphic conditions have not been altered substantially. No tendency for increased sedimentation upstream from bridges was observed. Deposition rates were inversely correlated with elevation and degree of ponding. Downstream deposition of sand splays appears to be related to flow constrictions and may be extensive. Mean overall rates of sedimentation (between 0.24 and 0.28 cm year−1), determined dendrogeomorphically, are comparable with other published rates.

A dendrogeomorphic approach to measurement of sedimentation in a forested wetland, Black Swamp, Arkansas

Dendrogeomorphic techniques were used to describe and interpret the spatial and temporal patterns of sedimentation in the Black Swamp, located along the Cache River in eastern Arkansas. At 30 sites along four transects, 148 trees were examined for depth of sediment accretion and cored for age determination. Tree-ring and geomorphic analyses indicated that mean sedimentation rates were significantly related to site elevation, topographic position, and the distribution of tupelo gum and bald cypress. Maximum mean rates of sedimentation, as high as 0.60 cm/yr, generally occurred in sloughs, areas low in elevation, and where tupelo gum and bald cypress grow. Mean sedimentation rate appeared to have a negative exponential relation with increasing elevation. Tree-agc class data indicated a significant increase in sedimentation rate since about 1945, from a mean at or below 0.01 cm/yr to a mean of 0.28 cm/yr for the past 19 years. Dendrogeomorphic techniques provide valid estimates of sedimentation rate and allow for the integration of decades of depositional processes in the rate calculation.