Jeffrey F. Mount

Bank Erosion as a Desirable Attribute of Rivers

ABSTRACT 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.

Mixing of siliciclastic and carbonate sediments in shallow shelf environments

The inhibiting effect that siliciclastic material has on carbonate-secreting organisms has lead to the generalization that sediments composed of mixtures of carbonate and siliciclastic material should rarely form. However, many modern and ancient shelf deposits contain a spectrum of sediments that are of “mixed” composition. The processes responsible for this mixing can be grouped into four categories: (1) punctuated mixing, where sporadic storms and other extreme periodic events transfer sediments from one depositional environment to another; (2) facies mixing, where sediments are mixed along the diffuse boundaries between contrasting facies; (3) in situ mixing, where the carbonate fraction consists of the autochthonous or parautochthonous death assemblages of calcareous organisms that accumulated on or within siliciclastic substrates; and (4) source mixing, where admixtures are formed by the uplift and erosion of nearby carbonate source terranes. The allochemical constituents of mixed sediments are both coralgal and foram-mollusc in composition. The foram-mollusc assemblage is the most common because of the effects of increased turbidity, unstable substrates, and the clogging of filter-feeding mechanisms associated with a siliciclastic influx.

Restoration of floodplain topography by sand-splay complex formation in response to intentional levee breaches, Lower Cosumnes River, California

Restoration of sustainable geomorphic processes that create floodplain topography through development of sand-splay complexes at intentional breaches is one method to promote variability in physical structure needed for habitat restoration. The topography of splay complexes provides a range of floodplain elevations that creates local variability in (i) inundation duration and frequency and depth to ground water that influence riparian vegetation establishment; and (ii) flow depth and velocity that create refuge for fish. Two intentional levee breaches along the lowland Cosumnes River, Central Valley, CA, were evaluated during water years 1999 and 2000 in order to document changes in morphology and relief associated with deposition of sandsplay complexes. During the study period, annual peak-flow recurrence intervals ranged from 1 to 3 years, and water flowed through the breaches for a minimum of 55 days during water year 1999 and 53 days during water year 2000. At the two study sites, rapid vertical accretion and scour occurred within the first several years after intentionally breaching the levee at the Accidental Forest floodplain (constructed in 1995) and at the Corps Breach floodplain (constructed in 1997). Splay complexes are organized into a variety of landforms, including lateral levees and lobes separated by new floodplain channels. Maximum deposition measured on the splay surface is 0.36 m/year, while maximum scour in channels is 0.27 m/year. Juxtaposition of floodplain splay deposition and adjacent channel scour creates relief ranging from 1.6 to 0.25 m that decreases with distance from the breach and that becomes more pronounced over time as higher magnitude floods scour channels in the old floodplain sediment and deposit new sand and silt onto the surface of the splay. The ratio of splay complex height to depth of formative flow is estimated as 0.4. Progradation of main and secondary splay channels takes place by down-floodplain sand transport (25 m/ year maximum). Large wood recruited onto the floodplain through the breach promotes local scour and deposition that enhances topographic variability. At one of the study sites, initial grading of a low setback berm prior to opening the breach forced a change in floodplain flow direction and the geometry of the splay complex. Additionally, progradation of the complex is arrested by an excavated pond that creates a sediment trap. We present a conceptual model that describes the importance of floods in constructing and modifying sand-splay complexes that create floodplain topography. The potential habitat variability created as floodplain topography evolves is the linkage between physical and ecological processes that are critical for restoration. D 2002 Elsevier Science B.V. All rights reserved.

Homogenous rivers, homogenous faunas

Habitat change and invasions of alien species are usually listed as the two biggest causes of biodiversity loss. When the two coincide, the results can be devastating, because altered habitat often favors alien species adapted to the changed conditions. This is particularly true in streams and rivers of North America, where homogenization of fish faunas through extinction of native species and invasions of alien species have been widely recorded (1). Although dams that alter flow regimes have been implicated as a causal factor behind this shift in North America, the degree to which flow regimes have been altered is not widely appreciated. The study by Poff et al. (2) in this issue of PNAS should help to change this problem. They document that flow alteration by dams in the U.S. is pervasive and that the resulting flow regimes tend to converge on one another, regardless of the original flow regimes of the dammed rivers.

Glacially Driven Cycles in Accumulation Space and Sequence Stratigraphy of a Stream-Dominated Alluvial Fan, San Joaquin Valley, California, U.S.A.

ABSTRACT High-resolution sequence stratigraphy provides a framework to interpret unconformity-bounded depositional sequences in the stream-dominated Kings River alluvial fan, located near Fresno, California. Depositional units in the fan are analogous to systems tracts described from marine deposits. Fan sequences reflect changes in accumulation space (Blum and Tornqvist 2000) associated with Pleistocene glacial cycles in the Sierra Nevada and preservation space created by tectonic subsidence in the San Joaquin basin. Adjustments in accumulation space are driven by changes in the ratio of sediment supply to discharge during glacial advances and retreats. At the end of glacial periods and the beginning of interglacial periods, declines in the ratio of sediment supply to discharge led to fan incision, a basinward shift in the fan intersection point, and loss of accumulation space. In mid- and upper-fan settings, incised valleys and laterally extensive, moderately mature paleosols formed, marking the unconformable base of the depositional sequence. Throughout the interglacial period, relatively low accumulation space existed and deposition was confined to the distal areas of the fan. Rapid aggradation and, thus, accumulation space increase, in response to increased sediment supply during the next glacial event initially filled the incised valley with a fining-upward succession of relatively coarse-grained channel and overbank deposits that contain rare, immature paleosols. Upon filling of the incised valley, the intersection point stabilized near the fan apex. This led to unconfined, open-fan deposition, indicating that widespread accumulation space was available across most of the fan surface. These high-accumulation-space units consist of fluvial deposits from multiple, large glacial outwash channels that radiated outward from the proximally located intersection point. Sequence boundaries and units associated with accumulation-space cycles can be used to understand and predict facies distributions and stratigraphic packaging within glacially influenced fans similar to the Kings River alluvial fan.

Hydrologic response and watershed sensitivity to climate warming in California's Sierra Nevada.

This study focuses on the differential hydrologic response of individual watersheds to climate warming within the Sierra Nevada mountain region of California. We describe climate warming models for 15 west-slope Sierra Nevada watersheds in California under unimpaired conditions using WEAP21, a weekly one-dimensional rainfall-runoff model. Incremental climate warming alternatives increase air temperature uniformly by 2°, 4°, and 6°C, but leave other climatic variables unchanged from observed values. Results are analyzed for changes in mean annual flow, peak runoff timing, and duration of low flow conditions to highlight which watersheds are most resilient to climate warming within a region, and how individual watersheds may be affected by changes to runoff quantity and timing. Results are compared with current water resources development and ecosystem services in each watershed to gain insight into how regional climate warming may affect water supply, hydropower generation, and montane ecosystems. Overall, watersheds in the northern Sierra Nevada are most vulnerable to decreased mean annual flow, southern-central watersheds are most susceptible to runoff timing changes, and the central portion of the range is most affected by longer periods with low flow conditions. Modeling results suggest the American and Mokelumne Rivers are most vulnerable to all three metrics, and the Kern River is the most resilient, in part from the high elevations of the watershed. Our research seeks to bridge information gaps between climate change modeling and regional management planning, helping to incorporate climate change into the development of regional adaptation strategies for Sierra Nevada watersheds.

Ecology and Management of the Spring Snowmelt Recession

We present a conceptual model for the ecology of the spring snowmelt recession based on the natural flow regime that relates the quantifiable components of magnitude, timing, and rate of change to abiotic and biotic factors that govern riverine processes. We find that shifts in the magnitude of the recession largely affect abiotic channel conditions, whereas shifts in the timing of the snowmelt primarily affect biotic conditions. Shifts in the rate of change affect both abiotic and biotic conditions, creating the largest observed changes to the stream ecosystem. We discuss these components with regard to the success of riverine species in Californias Mediterranean-montane environment. We then present two scenarios of change to the spring snowmelt recession—effects of flow regulation and climate warming—and discuss their potential implications for riverine ecology. Our conceptual model can help guide watershed stakeholders toward a better understanding of the impacts of changing spring recession conditions on stream ecosystems.

Petrology and geochemistry of rhizoliths from Plio-Pleistocene fluvial and marginal lacustrine deposits, east Lake Turkana, Kenya

ABSTRACT Recent studies of rhizoliths from the Koobi Fora Formation (Plio-Pleistocene) of East Lake Turkana, Kenya, indicate that their shapes vary with the depositional environment of their host sediment. Vertical rhizoliths are associated with channel-bar and overbank deposits of fluvial origin. These rhizoliths represent the tap roots of phreatophytes that lived in well-drained, upland settings. Horizontal rhizoliths predominate in facies ascribed to beaches, lagoons, and floodplains associated with ancient Lake Turkana. They are produced by water-emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments. The rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems. The vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements. These attributes reflect precipitation of the calcite in well-oxygenated, well-drained vadose conditions. In contrast, the cements of the horizontal root systems lack textures indicative of vadose conditions, contain abundant clay and plant debris, and display Mn concentrations as high as 4.5 cation percent. These features confirm that the horizontal rhizoliths developed in chemically reduced, water-saturated conditions. Bacterial decay of the plant debris maintained reduced Eh conditions while also supply ng abundant Mn2+ to the pore solutions. Groundwater that had moved through volcanic strata may have also contributed Mn2+. The Mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of Mn2+ as well as by inclusion of manganese oxides.

SIMULATED CHANGES IN SHALLOW GROUNDWATER AND VEGETATION DISTRIBUTIONS UNDER DIFFERENT RESERVOIR OPERATIONS SCENARIOS

The objectives of this study were to develop and use a linked groundwater and vegetation model to simulate groundwater and vegetation distributions in a riverine and reservoir-fringe system under different reservoir operations scenarios. This study was conducted where Little Stony Creek flows into East Park Reservoir on the east front of the Coast Range, northern California. A numerical groundwater model was used to model mean depth to groundwater during the growing season for water years 1980-1999 for each of five community types identified on the study site. Multiple vegetation models were devel- oped, each of which described the probability that a given community type would occur primarily as a function of modeled mean depth to groundwater during the growing season and secondarily as a function of flooding. Four scenarios representing four different res- ervoir operations were simulated: existing condition, existing condition with late drawdown, full drawdown, and full pool. A groundwater backwater effect caused by the imposed reservoir stage extends to portions of the terrace, but the most pronounced effects occur on the delta. Consequently, the most pronounced changes in vegetation distributions also occur on the delta. Compared to the existing-condition scenario, modeled vegetation dis- tributions do not change under the existing condition with late-drawdown scenario, a xeric herbaceous community type is greatly expanded under the full-drawdown scenario, and mesic herbaceous, scrub-shrub, and forested community types are greatly expanded under the full-pool scenario. The results of this study are twofold. First, the linked groundwater and vegetation model is relatively simple to construct and can be used to efficiently simulate multiple surface-water and groundwater management scenarios. Second, changes in res- ervoir operations can have pronounced effects on shallow groundwater and associated vegetation distributions in riverine and reservoir-fringe systems. Thus, the effects of chang- ing reservoir operations must be considered if the management of shallow groundwater and associated plant and wildlife habitat resources is to be successful.

Changes in lowland floodplain sedimentation processes: pre-disturbance to post-rehabilitation, Cosumnes River, CA

During the late Holocene, sediment deposition on the lowland Cosumnes River floodplain, CA has depended on factors that varied temporally and spatially, such as basin subsidence, sea level rise, flow, and sediment supply from both the Sacramento River system and from the Cosumnes River system itself, and anthropogenic changes. Through field investigations and analyses of historical maps, bridge core logs, and sediment size distributions, we link hydrogeomorphic processes to three stages of floodplain sedimentation on the lowland Cosumnes River. Stage I (1000–200 YBP) combined late Holocene pre-disturbance flood basin overflow and anastomosing river processes deposited spatially variable sediment consisting of gray–blue clay (87% clay) interlayered with relatively thin coarser sediment. Pre-disturbance Holocene deposition rates of up to f3.0 mm/year kept pace with sea level rise and tectonic basin subsidence. Stage II (200 to f10 YBP) anthropogenic disturbances caused a rapid increase in floodplain sedimentation rates up to 25 mm/year between 1849 and f1920, and deposited a relatively coarser reddish-brown sandy clay (f40% clay) layer that overlies the basin deposits. Between f1920 and 1990 AD, sedimentation was greatly limited on the lower Cosumnes floodplain because levees inhibited connectivity between both the Sacramento and Cosumnes River systems and the Cosumnes floodplain. During this stage, the density of channel segments in the anastomosing river floodplain decreased by 30% as agricultural activities filled secondary channels and leveled floodplain topography. During Stage III (f10 YBP to the present), post-rehabilitation floodplain sand splay complex sediment deposited after 1998 AD resulted from intentionally breaching levees to promote habitat at the Cosumnes River Preserve. The splay complex is dominated by medium to very coarse sand with finer intervening layers. The post-rehabilitation splay complex overlies the older basin deposits in a generally upward coarsening sequence that reflects depositional processes and land use changes that continue to affect the lowland Cosumnes River floodplain. D 2003 Elsevier Science B.V. All rights reserved.