Net Aggradation Research Articles

Managed Flood Effects on Beaver Pond Habitat in a Desert Riverine Ecosystem, Bill Williams River, Arizona USA

The ecological effects of beaver in warm-desert streams are poorly documented, but potentially significant. For example, stream water and sediment budgets may be affected by increased evaporative losses and sediment retention in beaver ponds. We measured physical attributes of beaver pond and adjacent lotic habitats on a regulated Sonoran Desert stream, the Bill Williams River, after ≥11 flood-free months in Spring 2007 and Spring 2008. Neither a predicted warming of surface water as it passed through a pond nor a reduction in dissolved oxygen in ponds was consistently observed, but bed sediment sorted to finest in ponds as expected. We observed a river segment-scale downstream rise in daily minimum stream temperature that may have been influenced by the series of ~100 beaver ponds present. Channel cross-sections surveyed before and after an experimental flood (peak flow 65 m3/s) showed net aggradation on nine of 13 cross-sections through ponds and three of seven through lotic reaches. Our results indicate that beaver affect riverine processes in warm deserts much as they do in other biomes. However, effects may be magnified in deserts through the potential for beaver to alter the stream thermal regime and water budget.

A Quaternary uplift record for the Auckland region, North Island, New Zealand, based on marine and fluvial terraces

Marine and fluvial terrace sequences near the Waitakere Ranges on the North Island of New Zealand have been surveyed, yielding an inventory of 13 fluvial and 12 marine terrace levels. Based on sparse tephra age control and correlation with the global palaeoclimatic record, rates of regional Quaternary uplift have been reconstructed. Between 1000 ka and 345 ka the time-averaged uplift rate was 0.072 mm a − 1 , between 345 ka and 50 ka it increased to 0.278 mm a − 1 , accelerating to 0.42 mm a − 1 since 50 ka. The fluvial terrace sequence did not yield clear sedimentary records or other datable material. However, although others have disputed the existence of marine terraces in this study region, a pattern of accelerating regional uplift, superimposed onto glacio-eustatic sea-level changes, is substantiated as the only possible mechanism for maintaining the considerable relief and the active denudation processes inland. The observed uplift is similar to that in other regions where the uplift has been attributed to coupling between surface processes and lower-crustal flow, making this a likely mechanism in the North Island of New Zealand. Regarding the fluvial terrace sequence, the proposed general model is of an actively incising river, carving out on average one strath terrace every ~ 16,000 years. The incision phases are reactivated by sea-level lowering and interrupted by net aggradation events due to landslides triggered by cyclones and/or fires within the catchment; volcanic ash falls also cause transient increases in sediment supply.

Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming

We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.

Hydrologic history of the Mississippi and Lower Missouri Rivers based upon a refined specific‐gauge approach

AbstractA refined specific‐gauge approach was developed to quantify changes over time in hydrological response on 3260 km of the Mississippi River system using long‐term data observed at 67 hydrologic measurement stations. Of these stations, 49 were unrated (stage‐only) stations, for which over 2 000 000 ‘synthetic discharges’ were generated based on measured discharge values at nearby rated stations. The addition of these synthetic discharges nearly tripled the number of stations in the study area for which specific‐gauge analysis could be performed. In order to maintain spatial homogeneity across such a broad study area, discharges were normalized to multiples of mean daily flow (MDF). Specific‐gauge analysis calculates stage changes over time for invariant discharge conditions. Two discharges were analysed: low‐flow and flood conditions at each station. In order to avoid the large errors associated with extrapolation of annual rating curves, a new ‘enhanced interpolation’ technique was developed that calculates continuous specific‐stage time series, even for rare discharges. Thus enhanced, specific‐gauge analysis is a useful reconnaissance tool for detecting geomorphic and hydrologic trends over time. Results show that on the Middle Mississippi River and Lower Missouri River, flood stages increased at all stations in spite of widespread incision of the river bed. On the Lower Mississippi River, both low‐flow and flood stages decreased, mainly the result of artificial meander cutoffs in the late 1920s and 1930s, except downstream of Natchez, MS, where net aggradation was observed. On the Upper Mississippi River, the specific‐gauge trends were dominated by emplacement of navigational dams and impoundment of slackwater pools. On all four river reaches, these results document hydrologic responses to the different engineering toolkits used on the different portions of the Mississippi River system during the past 75–150 years. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Vertical sorting and the morphodynamics of bed form‐dominated rivers: A sorting evolution model

Existing sediment continuity models for nonuniform sediment suffer from a number of shortcomings, as they fail to describe vertical sorting fluxes other than through net aggradation or degradation of the bed and are based on a discrete representation of the bed material interacting with the flow. We present a new type of sediment continuity model that is based on a stochastic description of the bed surface rather than discrete bed layers. The model is aimed at conditions dominated by bed load transport wherein the river bed is fully covered by river dunes. Application of the model should be limited to spatial scales covering a significant number of bed forms. The resulting model, i.e., the sorting evolution model, is suitable for unsteady conditions, as it takes into account the time evolution of, for instance, vertical sorting in modeling net aggradation or degradation of the river bed. The present paper lists the various submodels of a morphodynamic model system to which the sorting evolution model is applied. We compare the results of the morphodynamic model system to measured data from two flume experiments. The new model shows reasonable results for the predicted time evolution of the vertical sorting profile, as well as the time evolution of the grain size distribution of the bed load transport. Yet the model does not properly include sorting mechanisms associated with partial transport and the winnowing of fines from the trough surface and subsurface. The model serves as a basis for a future simplification of the model into a new stochastics‐based bed layer type sediment continuity model in which vertical sediment fluxes are included in a parameterized way.

Debris flow deposition and reworking by the Colorado River in Grand Canyon, Arizona

Flow regulation by large dams affects downstream flow competence and channel maintenance. Debris flows from 740 tributaries in Grand Canyon, Arizona, transport coarse‐grained sediment onto debris fans adjacent to the Colorado River. These debris fans constrict the river to form rapids and are reworked during river flows that entrain particles and transport them downstream. Beginning in 1963, flood control operations of Glen Canyon Dam limited the potential for reworking of aggraded debris fans. We analyzed change in debris fans at the mouths of 75‐Mile and Monument Creeks using photogrammetry of aerial photography taken from 1965 to 2000 and supplemented with ground surveys performed from 1987 to 2005. Our results quantify the debris fan aggradation that resulted from debris flows from 1984 to 2003. Volume, area, and river constriction increased at both debris fans. Profiles of the two debris fans show that net aggradation occurred in the middle of debris fans at stages above maximum dam releases, and surface shape shifted from concave to convex. Dam releases above power plant capacity partially reworked both debris fans, although reworking removed much less sediment than what was added by debris flow deposition. Large dam releases would be required to create additional reworking to limit the rate of debris fan aggradation in Grand Canyon.

Large‐scale fluvial grade as a nonequilibrium state in linked depositional systems: Theory and experiment

A graded river conveys its sediment load without net aggradation or degradation. Grade is thought to represent the equilibrium state of a river system subject to steady allogenic forcing. We address the concept of grade in linked depositional systems through a combination of mathematical modeling and flume experiments. We develop a moving boundary morphodynamic model of fluviodeltaic sedimentation and use it to quantify the conditions necessary for a state of sustained grade in an alluvial‐deltaic system developing on a uniformly sloping shelf. To test our theory, we conducted a series of flume experiments in which we constructed laboratory‐scale fluviodeltaic systems under specific relative sea level histories. On large spatiotemporal scales, our theoretical and experimental results suggest an alternative view of grade as an intrinsically nonequilibrium state that requires a fall in relative sea level. Under conditions of stillstand or rise in relative sea level, the equilibrium (graded) profile does not exist, and the alluvial river will always aggrade. With a steady sediment supply, alluvial grade can be sustained if relative sea level falls with a specific, square‐root‐of‐time dependence. Common sequence‐stratigraphic conceptualizations of alluvial river response to sea level change suggest that rivers aggrade and degrade in response to relative sea level rise and fall, respectively. We show that rivers can remain aggradational if the rate of sea level fall is sufficiently small. The boundary between aggradational and degradational states is the graded state described here: that associated with a relative sea level fall that varies with the square root of time.

Changes in the water surface profile of the Colorado River in Grand Canyon, Arizona, between 1923 and 2000

In 1923, a U.S. Geological Survey expedition surveyed the water surface profile of the Colorado River through Grand Canyon with theodolite and stadia rod. In 2000, lidar overflights collected topographic data centered on the river corridor, allowing construction of a new water surface profile and detection of change in the profile since 1923. By registering the surveys with respect to each other on the basis of 11 locations that were independently determined to have been unchanged between 1923 and 2000, 80 rapids were directly compared for change between 1923 and 2000. The average change for all measured rapids was +0.26 m, indicating net aggradation of the coarse‐grained alluvium forming the rapids throughout Grand Canyon. In addition, comparison of the two water surface profiles showed enhanced pool‐and‐rapid morphology. While 50% of the total drop of the river occurred in just 9% of the river distance in 1923, that value increased to 66% by 2000.

The micro‐topography of the wetlands of the Okavango Delta, Botswana

AbstractThe surface of the 40 000 km2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This micro‐topography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro‐topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro‐topography is, therefore, an expression of the non‐random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro‐topography, combined with estimates of current sediment influx, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near‐perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro‐topography has several useful hydrological applications. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Depositional and tectonic setting of the Miocene Wood Mountain Formation, southern Saskatchewan

Coarse gravels of the Miocene Wood Mountain Formation that cap the Wood Mountain upland in southern Saskatchewan are an erosional remant of a much more expansive sheet of gravel and sand. The formation consists of gravel and sand deposited as a thin sheet on a regional planation surface that extends approximately along strike for at least 160 km. This sheet, with a maximum thickness of 31 m, is the deposit of low-sinuosity, braided-river channels that flowed to the north-northeast. The Wood Mountain Formation represents an initial episode of erosion followed by low-accommodation deposition, where the net aggradation rate was likely low. The sediments of the Wood Mountain Formation are associated with change in tectonic style in western North America from one of compression to one of extension. The gravels were deposited above a regional erosion surface associated with isostatic uplift of the central Montana Uplift.

Airborne dust deposition in the Okavango Delta, Botswana, and its impact on landforms

AbstractThis study investigated the local‐scale generation and movement of dust in the seasonal swamps of the Okavango Delta, Botswana, with a view to examining possible transfer of material between flood plains and islands. It was found that most of the dust load was carried in the lowest 3 m of the air column, and consisted mainly of amorphous silica, indicating that dust was generated largely on the flood plains. Dust loads were found to be highest above the flood plains and lowest over the interiors of islands, probably due to the baffling effect of the island trees on wind velocity. The contrast in dust loads between islands and flood plains suggests that there is a net transfer of dust from flood plains to islands, but it was not possible to quantify this transfer. It is evident, however, that flood plains experience net erosion and islands net aggradation. A strong seasonality in dust loads was observed, with the maximum dust loads coinciding with maximum wind velocity in October. This also coincides with peak seasonal flooding in the delta, and only non‐inundated flood plains are capable of generating dust. Years of low flood therefore appear to be more dusty. There may also be transfer of material from higher‐lying to lower‐lying flood plains, which may reduce the topographic contrast on the flood plains. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Late Holocene Fluvial Change in a Tufa-depositing Stream: Davys Creek, New South Wales, Australia

Tufas are constructional landforms composed of calcium carbonate. They are common in karst terrains, and are known worldwide as useful archives of palaeoenvironmental information. In the case of fluvial tufas, these carbonate deposits experience net aggradation when environmental conditions are stable, whilst incision through the deposits occurs when conditions become wetter. In this study, the first of its type in temperate Australia, we used geomorphological and sedimentological evidence preserved in fossil tufas and alluvial terraces to investigate the fluvial history of Davys Creek (NSW, Australia). The nature and timing of fluvial changes along two quasi-independent reaches of the creek were synchronous, suggesting the operation of externally driven processes rather than local internal fluvial adjustments. Following a period of sustained aggradation, rapid incision occurred along the creek between 1500 and 1600 years before present ( bp ) in response to a shift to a moister climate. This was succeeded by a second major aggradation event (1500 years bp to 150 years bp ), then subsequent incision following the arrival of Europeans at 150 years bp.

Vertical sorting in bed forms: Flume experiments with a natural and a trimodal sediment mixture

Two sets of flume experiments were conducted to examine grain size selective transport and vertical sorting in conditions with migrating bed forms and bed load transport. In the two sets of experiments we used a sediment mixture from the river Rhine and a trimodal mixture, respectively. The vertical sorting profiles showed a downward coarsening trend within the bed forms, and in some experiments an essentially immobile coarse bed layer formed underneath the migrating bed forms. Three mechanisms contribute to the formation of such a coarse bed layer: (1) the avalanching process at the lee face, (2) conditions of partial transport in which a relatively large amount of coarse material does not participate in the transport process, and (3) the winnowing of fines from the trough surface and subsurface. The experiments show that vertical sorting fluxes not only occur through net degradation or aggradation but also through the migration of bed forms and through the variability in trough elevations. This is contradictory to the way vertical sorting processes are modeled in most existing sediment continuity models for nonuniform sediment. The present study is therefore also a plea for modifying existing sediment continuity models to account for vertical sorting processes other than through net aggradation or degradation.

Modelling the effect of vegetation on channel pattern in bedload rivers

AbstractWe modify a simple numerical stream‐pattern model to examine the effect of sediment stabilization by roots on the channel pattern of bedload rivers. In the model, vegetation enhances bank resistance to erosion, causing the development of a single channel instead of a rapidly changing, multiple channel (braided) pattern. Net aggradation resulting from a high sediment supply, however, causes frequent avulsions that destroy vegetation locally, leading to the development of a multiple‐channel pattern. A stability diagram representing multiple model runs predicts whether a river will exhibit single or multiple channels, based on plant‐enhanced bank strength, and on the time scale of plant development relative to a time scale for change in unvegetated channels. A second stability diagram predicts the way in which the amplitude and period of a fluctuating imposed sediment load influence whether a single or multiple‐channel pattern develops. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Sand sheet dynamics and Quaternary landscape evolution of the Selima Sand Sheet, southern Egypt

The Selima Sand Sheet occupies more than 120,000 km 2 of the hyperarid, uninhabited Darb el-Arba’in Desert centered at the border of Egypt and Sudan at latitude 22° N, and is characterized by a featureless surface of lag granules and fine sand broken only by widely separated dune fields and giant ripples of varying height and wavelength. Monitoring of the largest of these chevron-shaped ripples using repeat orbital images and field surveys indicates migration rates of 500–1000 m/yr, accompanied by 0–2.0 cm erosion or deposition of the youngest sand sheet stratigraphic units. Beneath this active surface, several developmental stages of sand sheet sediments have undulatory upper contacts and varying degrees of pedogenic alteration. The younger stages retain their horizontal lamination and have cracking patterns indicative of past wetter conditions, while older stages have lost their laminar structure through pedogenesis. Historical remains in the desert as well as 14C and Uranium-series dating indicate that the younger strata of the sand sheet have a very low accumulation rate, despite the active movement of the surface. The lower strata were extensively modified during mid and late Pleistocene pluvials, resulting in an initial undulatory surface that set the stage for later accumulation of sand sheet. Below these Quaternary sediments lies irregular topography dissected by channels of mid-Tertiary drainage. The Selima Sand Sheet is neither the result of net aggradation nor degradation, but results from inheritance of an initial fluvial landscape increasingly modified during climatic cycles. Wet periods led to local drainage and deposition, while the increasingly severe arid periods of the late Pleistocene and Holocene resulted in deposition of the blanketing bimodal sediments of the sand sheet.

Modeling longitudinal-profile development in response to Late Quaternary tectonics, climate and sea-level changes: the River Meuse

We present a forward-modelling case study for the development of the longitudinal profile of a basin–marginal fluvial system (River Meuse, NW Europe) responding to Late Quaternary tectonics, climate and sea-level changes. Modelling results show that river reaches in different tectonic domains will respond differently to isochronous events of climate and sea-level change. Climatic change dominates river–valley development in the long-term uplifting upper and middle Meuse reaches (NE Paris Basin, Ardennes, terrace flight area). An alternation of stadial depositional and interstadial erosional phases causes net aggradation of the river valley during glacials, but the river valley considerably degrades during interglacials. The long-term trend in these upper and middle reaches is river–valley degradation. Continuous aggradation leads to long-term deposition during glacials in the subsiding lower Meuse reaches, including the greater part of the Venlo-Block area, the western Roer Valley Graben and the southern part of the West Netherlands Basin. However, decreasing sediment supply combined with increasing discharges at the start of interglacials favours discharge-controlled (kinetic) incision in these net-depositional areas. Post-glacial sea-level rise and the subsequent interglacial highstand strongly influence the subsiding graben areas and the southern West Netherlands Basin. Rising sea level generates a zone of gradient-backfilling that migrates upstream with time. Consequently, the terrace intersection migrates upstream too and a coastal prism is built up during the interglacial highstand. The present case study indicates that the Meuse terrace intersection may migrate to at least 113 km upstream from the modelled coastline. The model also suggests post-interglacial incision of the Eemian highstand coastal prism during the subsequent relative sea-level fall in the Early Weichselian.

Glacial extent and catastrophic meltwater events during the deglaciation of Southern Iceland

Reconstruction of the paleoenvironment of South Iceland based on lithofacies and chronological studies of the Búdi morainal complex, lacustrine deposits at Ófærugil and a sediment core from Lake Hestvatn, South Iceland, revealed evidence for repeated catastrophic flood (jökulhlaup) activities during the late Younger Dryas (YD) and the early Preboreal (PB) chronozones. During this time, thin and lobate outlet glaciers from the retreating Iceland Ice Cap extended from major valleys and calved into the paleobay of southern lowlands of Iceland where relative sea level was at least 70 m higher than today. A number of ice-marginal lakes formed during the stepwise retreat of the South Iceland ice sheet at the transition of the YD and PB. Primary geomorphic and sedimentologic impact of the jökulhlaups was erosion of multiple channels downstream from the most prominent moraines in South Iceland and through the coastal sediment, accompanied by net aggradation of chaotically stratified sand and silt lenses. At the ice-distal environment, these floods accumulated in the form of recurrent turbidity currents. Volcanic eruptions, supplemented by a catastrophic release of the ice-marginal lakes, are postulated as the main triggers for the jökulhlaup activity. The jökulhlaup events evidenced in this study and subsequent break-up of the partly marine-based glacier may have contributed to the ice-rafting events found in the North Atlantic paleoenvironmental record.

Unconformities of the auriferous, neoarchæan central rand group of South Africa: application to stratigraphy

In addition to their importance as stratigraphical levels of enhanced Au concentration, the unconformities of the Central Rand Group of South Africa also form natural lithostratigraphical formation boundaries. With a view to stimulating research of these unconformities, two hypotheses on the origins of two types of unconformities are reviewed in this paper and criteria for their distinction are proposed. Progressive unconformities are syntectonic and diachronous in nature, and develop in response to source-area orogeny. The result of such tectonism is a basinward shift of the locus of erosion and a progradation, manifested by a conformable coarsening-upward succession in distal, downstream areas of net aggradation. However, in proximal areas net erosion prevails, and parts of such coarsening-upward successions are truncated. Progressive unconformities typically result from localised source-area tectonism and will therefore be restricted to the proximal regions of a particular sub-basin. Some unconformities develop due to eustatic changes and form during relative sea level falls. Because of differential subsidence within different parts of the composite Witwatersrand Basin, some such unconformities would be restricted to certain sub-basins. Both types of unconformities discussed form natural local lithostratigraphical boundaries. However, long-range correlation of formation boundaries is a practice alien to lithostratigraphy.

Gold dispersal and placer formation in an active oblique collisional mountain belt, Southern Alps, New Zealand

The Southern Alps of New Zealand constitute part of an orogenic belt forming on the western margin of the Pacific plate due to collision along the Pacific-Australian plate boundary. The topography of the mountain range is asymmetrical with a steep inboard zone adjacent to the plate boundary where very high rainfall causes rapid erosion, and a relatively gentle outboard zone in the rain shadow east of the topographic divide. Detritus, including gold, shed from the rising mountains are transported eastward from the outboard zone into a foreland basin, and westward from the inboard zone across the plate boundary to the Australian plate. Rivers in the outboard zone are widely spaced (>20 km) and structurally controlled oblique to the trend of the orogen. These rivers are long (>150 km) with relatively low gradients, and form braid plains showing net aggradation over the past million years. Gold placer formation in outboard rivers draining the central Southern Alps requires concentration factors of ca. 10 5 to 10 6 , which has occurred only during localized postdepositional uplift and recycling of sediments. Tectonic shortening coupled with low outboard erosion rates results in capture of some outboard rivers by inboard rivers, with consequent redirection of gold into the inboard region. Inboard rivers are generally short (20 km) and closely spaced (10 km), and oriented perpendicular to the plate boundary. Aggradation of sediments is temporary and periodic scouring of the valleys to bedrock by floods and glaciers results in ephemeral placer deposits in river bed lags and flood gravels on the Pacific plate. Gold is ultimately transported across the plate boundary on to the indentor (Australian) plate, in till or river gravels, where it is concentrated in degradational lags in fluvial sediments and in marine beach placers. Dextral tectonic motion at the plate boundary has transported detrital gold up to about 180 km laterally from Southern Alps sources during the past 5 m.y.

Paleosols in clastic sedimentary rocks: their geologic applications

Interest in pre-Quaternary paleosols has increased over the past decade, in large part, because they have proved to be beneficial in solving diverse geological problems. The majority of paleosols are described from continental deposits, most commonly from alluvial strata. Criteria for recognizing these paleosols have been extensively described; however, classifying them has proved more complicated. Pre-Quaternary paleosols are generally classified according to one or more modern soil classification systems, although one new classification has been proposed exclusively for paleosols to avoid problems using the modern soil classifications. In addition to taxonomic classification, paleosols can be categorized according to the interplay among deposition, erosion, and the rate of pedogenesis when they formed. Paleosols can be solitary if they formed during a period of landscape stability following the development of an unconformity. Such paleosols are commonly thick and extremely well developed. More commonly, paleosols are vertically stacked or multistory because they formed in sedimentary systems undergoing net aggradation. If erosion was insignificant and sedimentation was rapid and unsteady, compound paleosols generally formed. If the rate of pedogenesis exceeded the rate of deposition, composite paleosols developed. Thick, cumulative paleosols indicate that erosion was insignificant and that sedimentation was relatively steady. Both autogenic and allogenic processes can influence depositional and erosion patterns and, thus, affect the kinds of soils that form. Consequently, paleosols can help to interpret the history of sediment deposition and the autogenic and allogenic processes that influenced a sedimentary basin. Paleosols are also helpful in stratigraphic studies, including sequence stratigraphic analyses. They are used for stratigraphic correlations at the local and basinal scale, and some workers have calculated sediment accumulation rates based on the degree of paleosol development. In addition to their stratigraphic applications, paleosols can be used to interpret landscapes of the past by analyzing paleosol–landscape associations at different spatial scales, ranging from local to basin-wide in scope. At the local scale, lateral changes in paleosol properties are largely the result of variations in grain size and topography. At the scale of the sedimentary basin, paleosols in different locations differ because of basinal variations in topography, grain size, climate, and subsidence rate. Paleosols are used to reconstruct ancient climates, even to estimate ancient mean annual precipitation (MAP) and mean annual temperature (MAT). Ancient climatic conditions can be interpreted from modern soil analogs or by identifying particular pedogenic properties that modern studies show to have climatic significance. Stable carbon and oxygen isotopes are also used to interpret ancient climate, and some effort has been made to estimate MAT from isotopic composition. On the basis of modern soil analogs, paleo-precipitation has been estimated from the depth at which calcic horizons originally formed. Finally, paleosol carbonates have been used to estimate ancient atmospheric CO 2 values.