Terrace Surfaces Research Articles

Direct imaging of quantum wires nucleated at diatomic steps

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place preferentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step.

Surface Diffusion of a Zn Adatom on a Zn(001) Surface: A DFT Study

The adsorption and diffusion of a Zn atom on a Zn(001) surface has been investigated theoretically by using first-principles periodic boundary condition calculations to elucidate the mechanism of crystal growth of zinc in a Zn surface. Three surface models, terrace, step, and kink, of the Zn(001) surface were considered as adsorption sites in the present study. The most stable site in the terrace model was the on-top site, where the zinc adatom is bound to only one zinc atom of the surface. The activation barrier between the on-top site and the next on-top site was negligibly low (the activation energy is 44 meV at the PW91/LANL2DZ level), suggesting that the Zn adatom diffuses easily on the Zn(001) surface. The Zn adatom was more stabilized at the step site (752 meV) and kink site (935 meV) with respect to the on-top site. It was found that the magnitude of 4s−4p orbital mixing of the Zn adatom (hybridization) is strongly related to the binding energy. The mechanism of the adsorption of Zn on Zn(001) surfaces was discussed on the basis of theoretical results.

Late Quaternary uplift rate of the northeastern Japan arc inferred from fluvial terraces

Crustal uplift rates of the Isawa upland in the central part of the northeastern Japan forearc are estimated using the height distribution of late Quaternary fluvial terraces. The ages of fluvial terraces are correlated to marine isotope stages (MISs) by tephrochronology and 14C dating. The latest and older fill terrace surfaces were correlated to MIS 2/1 and MIS 6/5, respectively. The incision rate, estimated as 0.15–0.19 ± 0.07 m ky − 1 from the relative heights of the fill terrace surfaces, agrees roughly with the uplift rate along the Pacific coast (0.14–0.23 m ky − 1 ) determined from an MIS 5.5 marine terrace surface. In contrast, geodetic surveys (leveling surveys) and GPS observations show that the forearc is currently subsiding at a maximum rate of 5.6 mm y − 1 . Thus, the current geodetic subsidence does not reflect long-term (geological) tectonic movement. Vertical deformation of the northeastern Japan forearc is unique in terms of its direction and rate, as a deformation process of an overriding plate in an erosion-dominant subduction zone.

Formation of In-(2×1) and In islands on Si(100)-(2×1) by femtosecond pulsed laser deposition

The growth of indium on a vicinal Si(100)-(2×1) surface at room temperature by femtosecond pulsed laser deposition (fsPLD) was investigated by in situ reflection high-energy electron diffraction (RHEED). Recovery of the RHEED intensity was observed between laser pulses and when the growth was terminated. The surface diffusion coefficient of deposited In on initial two-dimensional (2D) In-(2×1) layer was determined. As growth proceeds, three-dimensional In islands grew on the 2D In-(2×1) layer. The RHEED specular profile was analyzed during film growth, while the grown In islands were examined by ex situ atomic force microscopy. The full width at half maximum of the specular peak decreased during the deposition, indicative of well-ordered growth and an increase of the island size. The In islands developed into elongated-polyhedral, circular, and triangular shapes. The elongated and triangular islands were highly oriented, parallel and perpendicular to the surface terrace edges, while the circular islands show a top flat surface. Deposition of In on Si(100)-(2×1) by fsPLD influenced the formation of the initial In-(2×1) layer and the morphology of the grown islands.

Exact linear response of reacting thermal defects driven by creation processes

The exact, linear response at steady state is calculated for reacting, but otherwise noninteracting, thermal defects driven by defect creation processes. The theory applies to vacancies and interstitials in the bulk, or to adatoms and advacancies on surface terraces. A wide variety of possible driving forces includes nuclear reaction, particle irradiation, epitaxial growth, surface erosion, and sublimation. When the defect life cycle typically starts and ends with spontaneous pair creation and annihilation, both species respond to the difference of their separate driving terms (the ``Poisson'' regime), and the law of mass action holds everywhere with a position dependent chemical potential ${\ensuremath{\mu}}^{*}(\mathbf{r})$. The value of ${\ensuremath{\mu}}^{*}(\mathbf{r})$ in linear response is employed here to discuss the conditions under which thermal defects precipitate, particularly as islands on terraces and dislocation loops in the bulk. It is shown, for the Poisson regime, that an approximate symmetry exists between processes for the two antidefects. Specifically, if ${\ensuremath{\mu}}_{c}^{*}$ suffices to nucleate a precipitate of one antidefect, then $\ensuremath{-}{\ensuremath{\mu}}_{c}^{*}$ is required to nucleate the other.

Structural properties of GaN grown on Zn-face ZnO at room temperature

We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 °C, and compared their structural properties. Although the film quality of GaN grown at 700 °C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 °C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 °C has a Ga-polarity.

Growth temperature dependence of structural properties of AlN films on ZnO (0001¯) substrates

The authors have investigated substrate temperature dependence of the structural properties of AlN grown on atomically flat ZnO (0001¯) substrates using pulsed laser deposition. The growth of AlN on ZnO at above 500°C proceeds in a three-dimensional mode and results in the formation of films with poor crystallinity and rough surfaces. However, reduction in the growth temperature leads to a remarkable improvement in crystalline quality and surface morphology. The authors have found that AlN films with stepped and terraced surfaces grow in the layer-by-layer mode at substrate temperatures below 300°C. X-ray reflectivity measurements have revealed that these improvements in structural properties can be attributed to the suppression of the interfacial reactions between AlN and ZnO.

FLOODPLAIN VEGETATION AND SOILS ALONG THE UPPER SANTA ANA RIVER, SAN BERNARDINO COUNTY, CALIFORNIA

The patterns of vegetation and soils were documented in an approximately 20 km2 area of fluvial terraces adjacent to the Santa Ana River in southwestern San Bernardino County, California. Within this area there are three terraces of differing elevations that were last disturbed during major flood events. The oldest terrace surfaces probably were last disturbed during the Agua Mansa flood of 1862, which disturbed or created many current geomorphic features of the Santa Ana River basin. The most recent disturbance of two other terraces was identified based on photographs of flooding events in 1938 and 1969. Principal component analysis identified three assemblages of species whose distribution corresponded to the three terraces that differed in elevation, soil texture, and age since last disturbance by flooding. Canonical correspondence analysis showed that the assemblages identified by PCA were highly correlated with changes in soil texture and organic matter. The most reliable indicator species were Heterotheca sessiliflora and Lepidospartum squamatum for the first assemblage (associated with the lowest terrace, and corresponding to early successional assemblages identified by other researchers); Salvia apiana and Senecio flaccidus for the second assemblage (associated with the intermediate terrace); and Artemisia californica, Opuntia parryi, and Stephanomeria pauciflora for the third assemblage (associated with the highest terrace). Eriastrum densifolium subsp. sanctorum, the rare Santa Ana River Woolly Star, was associated with the earlier phases of succession. The most important soil factor distinguishing these assemblages was the silt/clay content of the soil.

Geomorphic constraints on listric thrust faulting: Implications for active deformation in the Mackenzie Basin, South Island, New Zealand

Deformed fluvial terraces preserved over active thrust‐related folds record the kinematics of folding as fault slip accumulates on the underlying thrust. In the Mackenzie Basin of southern New Zealand, the kinematics revealed by folded fluvial terraces along the active Ostler and Irishman Creek fault zones are inconsistent with traditional models for thrust‐related folding in which spatially uniform rock uplift typically occurs over planar fault ramps. Instead, warped and tilted terraces in the Mackenzie are characterized by broad, continuous backlimbs and abrupt forelimbs and suggest folding through progressive limb rotation. By relating this pattern of surface deformation to the underlying thrust with a newly developed, simple geometric and kinematic model, we interpret both faults as listric thrusts rooted at depth into gently dipping planar fault ramps. Constraints on the model from detailed topographic surveying of deformed terraces, ground‐penetrating radar over active fault scarps, and luminescence dating of terrace surfaces suggest slip rates for the Ostler and Irishman Creek faults of ∼1.1–1.7 mm/yr and ∼0.5–0.7 mm/yr, respectively. The predicted depth of listric faulting for the Ostler fault (0.7−0.2+0.1km) and the Irishman Creek fault (1.3−0.5+0.1km) generally agrees with geophysical estimates of basin depth in the Mackenzie and suggests control of preexisting basin architecture on the geometry of active thrusting. Despite the potential effects of changes in fault curvature and hanging wall internal deformation, the methodology presented here provides a simple tool for approximating the kinematics of surface deformation associated with slip along listric, or curviplanar, thrust faults.

Geoarchaeology of the Kostenki–Borshchevo sites, Don River Valley, Russia

AbstractThe Kostenki–Borshchevo localities include 26 Upper Paleolithic sites on the first and second terraces along the west bank of the Don River, near Voronezh on the central East European Plain. Geoarchaeological research from 2001 through 2004 focused on sites Kostenki 1, 12, and 14, with additional work at Kostenki 11 and 16, and Borshchevo 5. The strata are grouped into three units (bottom up): Unit 1, > 50 ka, consists of coarse alluvium (representing upper terrace 2 deposits) and colluvium, overlain by fine‐grained sediments. Unit 2 includes archaeological horizons sealed within two sets of thin lenses of silt, carbonate, chalk fragments, and organic‐rich soils (termed the Lower Humic Bed and Upper Humic Bed) dating 50–30 ka. Separating the humic beds is a volcanic ash lens identified as the Campanian Ignimbrite Y5 tephra, dated elsewhere by Ar/Ar to ca. 40 ka. The humic beds appear to result from the complex interplay of soil formation, spring deposition, slope action, and other processes. Several horizons buried in the lower part of Unit 2 contain Upper Paleolithic assemblages. The springs and seeps, which are still present in the area today, emanated from the bedrock valley wall. Their presence may account for the unusually high concentration of Upper Paleolithic sites in this part of the central East European Plain. Unit 3, < 30 ka, contains redeposited loess with a buried soil (Gmelin Soil) overlain by a primary full‐glacial loess with an associated Chernozem (Mollisol), forming the surface of the second terrace. © 2007 Wiley Periodicals, Inc.

Pedogenic approach to resolving the geomorphic evolution of the Pakua river terraces in central Taiwan

The southern part of the Pakua tableland in Taiwan comprises a series of wide, unpaired river terraces that have as many as six altitude levels, from PK-1 (highest) to PK-6 (lowest). The terrace surfaces are covered with lateritic soils. This study described, sampled and analyzed the soils from each terrace. Soil taxonomies are Oxisol, Ultisol and Inceptisol . Soil morphological characteristics were quantified as a profile development index (PDI) to determine the degree of soil development. The chemical analyses included cation exchange capacity (CEC) and selected extraction of Fe in soils. Based on the degree of soil development, surface deposits on terrace flights form the post-incisive soil chronosequence, supporting the hypothesis of terrace formation by fluvial processes. Soil ages were estimated based on iron crystallinity (Fe d–Fe o)/Fe t. Analytical results indicate that river terraces in the Pakua tableland are aged 40–400 ka. The long-term uplift rate of the tableland can be estimated using terrace ages and folding geometry. The variation of uplift rates demonstrates that the landscape underwent increased shortening of frontal thrusting by the Changhua Fault running beneath the tableland.

Pleistocene tectonics inferred from fluvial terraces of the northern Upper Rhine Graben, Germany

This study of fluvial terraces of the River Rhine and tributaries aims to search for indications of Pleistocene tectonic activity. The study area includes the northern Upper Rhine Graben (URG), the Mainz Basin and the adjacent Rhenish Massif with the Middle Rhine Valley. High rates of Quaternary surface processes, large amount of human modifications, relatively slow tectonic deformation and presently low intra-plate seismic activity characterize this area. Therefore, the records of relatively slow tectonic deformation are less well preserved and thus difficult to detect. This study uses the relative position of fluvial terraces to determine the more local effects of fault movements on the terraces and to evaluate their displacement rates and patterns. The research is based on a review of previous terrace studies and new terrace mapping from the eastern Mainz Basin and the bordering URG using topographic map interpretations and field observations. This newly mapped sequence of terrace surfaces can be correlated to other terraces in the vicinity on the basis of relative height levels. Terrace correlation between the western Mainz Basin and Middle Rhine Valley relies on a single chronostratigraphic unit (Mosbach sands) and additional relative height correlations. This is the first study to present a continuous correlation of terraces from the western margin of the URG to the Rhenish Massif and enables the study of the transition from the subsiding graben to the uplifted Rhenish Massif. By means of a longitudinal profile, which ranges from the URG to the Rhenish Massif, the influence of individual fault movements on the terrace levels and the large-scale regional uplift is demonstrated. It is evident from the profile that the uplift of Early to Middle Pleistocene terraces increases northwards, towards the Rhenish Massif. The uplift was diachronic, with a significant pulse occurring first in the northern URG (Lower Pleistocene) and later in the Rhenish Massif (Middle Pleistocene). The largest vertical displacements are recorded for the boundary fault separating the Mainz Basin and the Rhenish Massif (Hunsrück–Taunus Boundary Fault) and for faults bounding the northeastern Mainz Basin. The motions and displacement rates calculated for individual faults indicate deformation rates in the order of 0.01–0.08 mm/year. At this stage, the calculation of displacement rates depends mostly on a single dated stratigraphic unit. Additional dating of terrace deposits is urgently needed to better constrain the temporal development of the terrace sequence and the impact of tectonic movements.

Analysis of titanomagnetite within weathered middle Pleistocene KMT tephra and its application for fluvial terrace chronology, Kanto Plain, central Japan

The widespread Kaisho-Kamitakara tephra (KMT), derived from the source vent located in the Hida Mountains, is a favorable marker tephra for the chronological study of the middle Pleistocene in the Kanto Plain, central Japan. As the KMT in aeolian sediments is strongly weathered, the chemical composition of titanomagnetite, resistant to weathering, was determined for its identification. The age of the KMT was estimated at 0.62 Ma (MIS 16.0) by Nakazato [2006. Horizon and age of Kaisho-Kamitakara tephra from the Inubo Group in the northeastern part of Chiba Prefecture, central Japan. Programme and Abstracts, Japan Association for Quaternary Research 36, 106–107 (in Japanese)], revising a previous estimate of 0.58–0.69 Ma by Suzuki [2000. Kaisho-Kamitakara tephra erupted from the Hida Mountains in early half of middle Pleistocene and its significance for geomorphic chronology of central Japan. Geographical Review of Japan 73, 1–25 (in Japanese)] The stratigraphic position of the KMT in the Azuyama and Sayama terrace surfaces in southwest Kanto and the Kitsuregawa Upper terrace surface in north Kanto indicates that these terraces are correlative. It is assumed that the dissected fluvial terrace surfaces were formed a few tens of thousands of years prior to the deposition of the KMT. Alluvial fan deposits constituting these terrace surfaces were formed during the period from MIS 17.3 to 16.2, during a transition from interglacial to glacial. Geomorphic conditions during this period of the middle Pleistocene were stable, allowing the formation of broad fans.

Effect of fluvial and geomorphic disturbances on habitat segregation of tree species in a sedimentation-dominated riparian forest in warm-temperate mountainous region in southern Japan

We investigated habitat segregation and patterns of species diversity of trees in relation to variations in fluvial and geomorphic disturbances (erosion or sedimentation) along a longitudinal stream gradient from V-shaped valley to alluvial fan and between valley and adjacent hill slopes in a warm-temperate mountainous riparian forest in Kyushu, southwestern Japan. We longitudinally divided the riparian area into four geomorphic zones: V-shaped valley (VV-zone), upper fan (UF-zone), middle fan (MF-zone), and lower fan (LF-zone). We surveyed the distribution of tree species (diameter at breast height ≥3 cm) in the four riparian zones and in additional plots on hill slopes (SL-zone) representing the broader, nonriparian forest matrix. Detrended correspondence analysis ordination demonstrated variations in species composition along the longitudinal stream gradient. Species guild analysis based on the detection of the species preferred zone by a bootstrap method revealed a guild structure corresponding to each geomorphic zone. The four riparian zones were differentiated from the SL-zone by having a low proportion of SL-guild species and a high proportion of infrequent species that were characterized by deciduous leaf habit. The LF-zone was the most differentiated and was characterized by low tree density and specialist species established on the flat and unstable soil surface created by frequent deposition of sediment. The UF- and MF-zones were characterized by a high tree density and species richness (particularly of infrequent species) established on the stable ground surface of a high alluvial terrace. Microsite heterogeneity produced by channel formation may also maintain a high species diversity in the riparian zones.

Ultra-High Resolution Imaging by Fluorescence Photoactivation Localization Microscopy

Biological structures span many orders of magnitude in size, but far-field visible light microscopy suffers from limited resolution. A new method for fluorescence imaging has been developed that can obtain spatial distributions of large numbers of fluorescent molecules on length scales shorter than the classical diffraction limit. Fluorescence photoactivation localization microscopy (FPALM) analyzes thousands of single fluorophores per acquisition, localizing small numbers of them at a time, at low excitation intensity. To control the number of visible fluorophores in the field of view and ensure that optically active molecules are separated by much more than the width of the point spread function, photoactivatable fluorescent molecules are used, in this case the photoactivatable green fluorescent protein (PA-GFP). For these photoactivatable molecules, the activation rate is controlled by the activation illumination intensity; nonfluorescent inactive molecules are activated by a high-frequency (405-nm) laser and are then fluorescent when excited at a lower frequency. The fluorescence is imaged by a CCD camera, and then the molecules are either reversibly inactivated or irreversibly photobleached to remove them from the field of view. The rate of photobleaching is controlled by the intensity of the laser used to excite the fluorescence, in this case an Ar+ ion laser. Because only a small number of molecules are visible at a given time, their positions can be determined precisely; with only ∼100 detected photons per molecule, the localization precision can be as much as 10-fold better than the resolution, depending on background levels. Heterogeneities on length scales of the order of tens of nanometers are observed by FPALM of PA-GFP on glass. FPALM images are compared with images of the same molecules by widefield fluorescence. FPALM images of PA-GFP on a terraced sapphire crystal surface were compared with atomic force microscopy and show that the full width at half-maximum of features ∼86 ± 4 nm is significantly better than the expected diffraction-limited optical resolution. The number of fluorescent molecules and their brightness distribution have also been determined using FPALM. This new method suggests a means to address a significant number of biological questions that had previously been limited by microscope resolution.

Room temperature epitaxial growth of AlGaN on ZnO by pulsed laser deposition

The authors have grown Al0.1Ga0.9N films on atomically flat ZnO substrates at room temperature (RT) by pulsed laser deposition. Epitaxial growth of AlGaN at RT proceeds in the layer-by-layer mode, and the films show atomically flat stepped and terraced surfaces. On the other hand, growth at 600°C proceeds three dimensionally, and the films suffer from degradation in their crystalline quality and from rough surface morphology. These results indicate that suppression of the formation of interfacial layers between AlGaN and ZnO by reducing the growth temperature is inherently important in order to take advantage of the nearly lattice-matched ZnO substrates. They have also found that high-quality AlGaN films can be obtained under highly N-rich conditions at reduced growth temperatures, which provides a striking contrast to the case of molecular beam epitaxy.

Calcrete ‘fossilisation’ of alluvial fans in SE Spain: The roles of groundwater, pedogenic processes and fan dynamics in calcrete development

Dryland alluvial fans developed along the northern flanks of the Sierra Lisbona in the north-western Vera basin (Almería region, southeast Spain) display negligible amounts of incision despite distal base-level lowering of > 30 m by the modern drainage network. The fans are of early-middle Quaternary age and are graded to a coeval river terrace, now isolated from the modern drainage network. The negligible incision is due to the encasement and ‘fossilisation’ of the alluvial fans by calcrete. This paper explores the reasons for such fossilisation and assesses the relative importance of pedogenic and groundwater mechanisms for calcrete formation within an alluvial fan setting. A single fan was selected for detailed examination. The geomorphological and sedimentological features of the fan, its catchment area and their relationship to the distal river terrace were documented. Qualitative and semi-quantitative petrographic and scanning electron microscope analyses of calcrete samples collected from transects across the fan surface, and within its distal top river terrace surface, enabled the style, pattern and relative timing of calcrete development to be assessed. Calcrete fabrics comprised initial micritic grain-coating cements, pellets and glaebular carbonate nodules, with interstitial spaces infilled by equant sparite and microsparite mosaics. It is proposed that the early phases of calcrete development were dominated by pedogenic processes with increasing groundwater calcretisation over time. Point count data indicated increased quantities of interstitial sparite and microsparite cement within near-surface proximal fan calcretes and at depth across the fan, suggesting that groundwater processes played a more important role in calcrete formation in these locations. The contribution of groundwater to calcrete development can be best explained by the intrinsic funnelling of groundwater from the catchment through the proximal fan head area, a zone where the fan gravels are thinnest. Calcrete ‘fossilisation’ appears to have followed a reduction in the fan catchment area as a result of rockfalls and watershed stream capture, which reduced water and sediment supply to the fan and enabled surface stabilization and calcrete development to take place. The reduced sediment/water supply, combined with calcrete fossilisation, appears to have protected the alluvial fans from regional base-level lowering. The implications of these results for existing pedostratigraphic models of calcrete development in alluvial fans are subsequently explored.

Microstructuring of stainless steel implants by electrochemical etching

The effects of electrochemically enhanced etching on stainless steel coronary stent surfaces have been investigated in respect to their applicability as surface modifications prior drug-coating. Two methods have been investigated, one basing on grain boundary etching with diluted HNO3 and the other one on hydrochloric acid etching. The etching current was in the range of 30–200 mA which accounts for 0.34–2.28 mA/mm² surface. Grain boundary etching produced a micro-furrowed surface providing volume for the coating drug. The theoretical volume offered by the furrows was calculated on the basis of laser perthometry and was determined to be 0.146 mm³/cm². With the hydrochloric acid etching method it was possible to generate an evenly rough, terraced surface. Both surfaces have been coated with Rapamycin in ethanol (20 mg/mL) and examined under SEM after dilatation. It was shown that a uniform drug layer is maintained after dilatation of the stent and little flaking is visible. Quantification of the amount of Rapamycin yielded 21.4 μg/mm² for the electropolished stents, 36.6 μg/mm² for the grain-boundary etched stents and 27.7 μg/mm² for the hydrochloric acid etching after dilatation. For the grain boundary etched stents an improved drug adhesion was found, while the hydrochloric acid etchings resulted in a deterioration of the adhesion properties.

High electrical conductivity of layered cobalt oxide Ca3Co4O9 epitaxial films grown by topotactic ion-exchange method

Epitaxial film of a layered cobalt oxide, Ca3Co4O9, was fabricated on a (0001) face of an α-Al2O3 substrate by a topotactic ion-exchange method using a γ-Na0.8CoO2 epitaxial film as a precursor. High-resolution x-ray diffraction and atomic force microscope measurements revealed that the film was high-quality (001)-oriented Ca3Co4O9 with stepped and terraced surface morphology. The film exhibits a high electrical conductivity of 2.95×102Scm−1 and a large Seebeck coefficient of ∼+125μVK−1, which leads to the thermoelectric power factor (TPF) of 4.5×10−4Wm−1K−2 at 300K, potentially usable as a building block of the multilayered film structure with an enhanced TPF value.

Predicting vertical accretion rates at an archaeological site on the Mississippi River floodplain: Effigy Mounds National Monument, Iowa

The Sny Magill Unit of Effigy Mounds National Monument, Iowa, contains the largest cluster of prehistoric effigy mounds on public land in North America. The mounds are situated atop a low terrace of the Upper Mississippi River, where they are slowly being buried by overbank deposition during floods. The terrace surface includes forest soils with argillic (Bt) or cambic (Bw) horizons developed in up to 1 m of loamy overbank deposits on top of Pleistocene sand and gravel. Radiocarbon evidence suggests that overbank deposits have accumulated since the end of the mound-building period (about 700 years BP), yielding a vertical accretion rate of about 0.6 mm yr − 1 . On the basis of 137Cs analysis, accretion rates over the past 40–50 years average 1.25–2.07 mm yr − 1 , with some evidence for a decreasing rate since 1964. If these accretion rates are projected forward, several of the effigy mounds could be buried by flood deposits within 150–300 years. This 137Cs-derived estimate agrees closely with an estimate of burial times based on flood frequency and observed flood deposit thickness during recent floods. However, the floodplain and backwater environments of the Upper Mississippi River are aggrading much more rapidly than the Sny Magill terrace surface, suggesting that burial of the entire terrace could occur within 80–400 years and the entire mound group could be buried within 150–850 years. The projected accretion rates and time to burial are subject to large uncertainties because of environmental change in the watershed, including recent trends toward increasing flood stages and decreasing suspended sediment loads.