Leal A. K. Mertes

Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil

Sediment transport through the Brazilian sector of the Amazon River valley, a distance of 2010 km, involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at Obidos (∼1200 Mt yr −1 ). The exchanges occur through bank erosion, bar deposition, settling from diffuse overbank flow, and sedimentation in flood-plain channels. We estimated the magnitude of these exchanges for each of 10 reaches of the valley, and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach. Residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel. The annual supply of sediment entering the channel from bank erosion was estimated to average 1570 Mt yr −1 (1.3 × the Obidos flux) and the amount transferred from channel transport to the bars (380 Mt yr −1 ) and the flood plain (460 Mt yr −1 in channelized flow; 1230 Mt yr −1 in diffuse overbank flow) totaled 2070 Mt yr −1 (1.7 × the Obidos flux). Thus, deposition on the bars and flood plain exceeded bank erosion by 500 Mt yr −1 over a 10–16 yr period. Sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 Mt yr −1 over 16 yr, crudely validating the process-based calculations of the sediment budget, which in turn illuminate the physical controls on each exchange process. Another 300–400 Mt yr −1 are deposited in a delta plain downstream of Obidos. The components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain, which in turn are influenced by tectonic features of the Amazon structural trough.

Channel-floodplain geomorphology along the Solimões-Amazon River, Brazil

Across the cratonic landscape of Brazil the Solimoes-Amazon River transports to its delta plain 1240 Mt of suspended sediment derived from Andean erosion and reworks another 3200 Mt of floodplain sediments. Distribution of these sediments has resulted in a variable along-stream pattern of geomorphology. The upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel. Sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll-bar topography that forms the boundaries of hundreds of long, narrow lakes. In contrast, downstream reaches are characterized by channels restricted by stabilizing, long-term, levee building and floodplain construction dominated by overbank deposition. Overbank deposition buries the scroll-bar topography, resulting in a flat floodplain covered by a patchwork of large, more equant, shallow lakes. On the basis of estimated rates of recycling of floodplain sediments, the modern floodplain of the Brazilian Amazon could have been recycled in <5000 yr, and is recycled more rapidly in the upstream than the downstream reaches. The cratonic interior is interrupted by structural arches that bound intracratonic basins. Four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel-floodplain geomorphology at this spatial scale. Structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation, thus restricting channel movement. For example, as the river crosses the Purus arch, the valley narrows to <20 km compared to an average of ∪45 km, the water-surface gradient decreases, sediment is deposited, and yet the rate of channel migration is negligible. Hence, the effect of the arches is to create a landscape where, on the spatial scale of hundreds of kilometres, the river is confined and entrenched in its valley, is straight, and is relatively immobile. Local valley tilting apparently unrelated to the arch structures also imprints the geomorphology. In particular, a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large-scale, oxbow-type features on the Brazilian Amazon River floodplain.

Spatial patterns of hydrology, geomorphology, and vegetation on the floodplain of the Amazon river in Brazil from a remote sensing perspective

Abstract The spatial heterogeneity of hydrology and vegetation during high-water periods in geomorphically distinct reaches of the Amazon River in Brazil was determined based on semivariance statistics. The spatial statistics were derived from three classified Landsat Thematic Mapper images representing upstream to downstream geomorphic characteristics. In the upstream river reach, scroll-bar topography on the floodplain tends to channelize floodwater into floodplain drainage channels, thus reducing the diversity of water types by reducing opportunities for mixing of flooding river water with locally derived floodplain water. The highest diversity of vegetation types is along floodplain drainage channels, while the rest of the floodplain has a more homogeneous cover. In the middle reach of the river the diversity of wetland classes as measured by semivariance is higher than both upstream and downstream, perhaps because of exposure to more water types and landforms. The diversity of water types is high, because flooding river water flows onto the floodplain as diffuse, non-channelized overbank flow, as well as through drainage channels. The non-channelized overbank flow readily mixes with locally derived floodplain water. Floodplain landforms available for colonization by vegetation include scroll bars, swales, lake shores, lake deltas, and floodplain drainage channels. In the downstream reach where the floodplain is wide, relatively flat, and covered with huge lakes, the floodplain supports a moderately heterogeneous mix of vegetation communities. Where landforms are similar, the spatial distribution of the vegetation is similar to that of the middle reach of the river. In the downstream reach flooded forest comprised only 37% of the wetland vegetation. In contrast, in both the upstream and middle reaches, over 70% of the wetland vegetation was flooded forest. Agricultural clearing of the floodplain is more. common in downstream reaches and may account for the smaller percent of floodplain forest cover.

Rates of flood-plain sedimentation on the central Amazon River

Field measurements of water and sediment discharge and floodplain topography, numerical simulations of hydrology, and Landsat data on sediment concentrations were combined to estimate rates of transport and the fate of water and sediment in a 200 km reach of the central Amazon River. Results from field data collected during the annual flood in July 1986 indicate that sediment was transported to the flood plain at rates ranging from 3 to 5 t ⋅ d -1 ⋅ m -1 , with percentage deposition >80% yielding vertical rates of 0.8 to 1.6 cm/day. Comparable rates were calculated from geographic information system overlays of numerically simulated specific water discharge and Land-sat-derived sediment concentrations. Rates for July 31, 1977, include 4 to 18 t ⋅ d -1 ⋅ m -1 of transport onto the flood plain and an average of 64% deposition at vertical rates from 0.4 to 3.3 cm/d. A 1 m higher flow on August 2, 1989, yielded, on average, higher rates of transport onto the flood plain, 8 to 15 t ⋅ d -1 ⋅ m -1 , but lower percentage (46%)deposition, implying vertical rates between 0.3 and 2.9 cm/d.

Measuring flood output from 110 coastal watersheds in California with field measurements and SeaWiFS

The 1300-km-long California coast receives output from watersheds that range from ∼10 km 2 within the coastal mountain ranges to ∼120 000 km 2 for the drainage basins of the Sacramento and San Joaquin Rivers. Results from a combination of field and remote-sensing data, modeling, and digital techniques show that during the El Nino winter of 1998, the impact of sediment and water transport on nearshore waters from coastal watersheds was spatially extensive, varied with depth, and was comparable to the impact of output from large rivers. The mass of sediment estimated from SeaWiFS data for 25 discrete and coalesced buoyant river plumes emanating from 110 watersheds was 2.2 × 10 6 t. Although the mass of all of the plumes represents only 1%–2% of the total sediment output from the rivers, the plumes covered an area of 29 500 km 2 of coastal waters. An important cautionary note is that the surface plumes visible in the remote-sensing data (even though nearly coincident with flooding) do not represent mass output from the rivers.

Estimating suspended sediment concentrations in turbid coastal waters of the Santa Barbara Channel with SeaWiFS

A technique is presented for estimating suspended sediment concentrations of turbid coastal waters with remotely sensed multi-spectral data. The method improves upon many standard techniques, since it incorporates analyses of multiple wavelength bands (four for Sea-viewing Wide Field of view Sensor (SeaWiFS)) and a nonlinear calibration, which produce highly accurate results (expected errors are approximately ±10%). Further, potential errors produced by erroneous atmospheric calibration in excessively turbid waters and influences of dissolved organic materials, chlorophyll pigments and atmospheric aerosols are limited by a dark pixel subtraction and removal of the violet to blue wavelength bands. Results are presented for the Santa Barbara Channel, California where suspended sediment concentrations ranged from 0–200+ mg l−1 (±20 mg l−1) immediately after large river runoff events. The largest plumes were observed 10–30 km off the coast and occurred immediately following large El Niño winter floods.

Geocoded digital videography for validation of land cover mapping in the Amazon basin

Four Validation Overflights for Amazon Mosaics (VOAM) aerial video surveys have been carried out in the Brazilian Amazon to provide ground verification for mapping of wetland cover with the Global Rain Forest Mapping (GRFM) Project JERS-1 (Japanese Earth Remote Sensing Satellite) mosaics of the Amazon basin. Surveys in 1995 and 1996, acquired with handheld analog camcorders from small aircraft, were timed to imaging of the GRFM low- and high-water mosaics, and limited to within 600 km of Manaus. For the 1997 and 1999 flights, digital camcorder systems were installed in the Bandeirante survey plane operated by Brazils National Institute for Space Research. The VOAM97 and VOAM99 surveys circumscribed the Brazilian Amazon, documenting ground conditions at resolutions on the order of 1 m (wide-angle format) and 10 cm (zoom format) for wetlands, forests, savannas, and human-impacted areas. Global Positioning System (GPS) information encoded on the video audio track was extracted by mosaicking software that automatically generates geocoded digital mosaics from video clips. On the 1999 survey, a laser altimeter recorded profiles of terrain and vegetation canopy heights. A validation dataset was compiled from the videography for a portion of the GRFM mosaics extending 6° by 4° in longitude and latitude, using randomly selected points along flight lines. Other applications of the VOAM videography include acquisition of ground control points for image geolocation, creation of a high-resolution geocoded mosaic of a forest study area, forest biomass estimation, and rapid assessment of fire damage. Geocoded digital videography provides a cost-effective means of compiling high-resolution validation datasets for land cover mapping in remote, cloud-covered regions.

Sediment characteristics of an extreme flood: 1993 upper Mississippi River valley

The 1993 Mississippi River flood was notable for its high magnitude, long duration, summer occurrence, and low sediment discharge. A field survey of a 70-km-long reach in the vicinity of Quincy, Illinois, revealed that the event was characterized by 100 yr flood had remarkably little sedimentological or geomorphological impact on the flood plain within the study reach because the transport effectiveness of floods in large drainage basins is influenced by event sequencing in the same manner as floods in small watersheds, and the cohesive flood-plain soils were not susceptible to erosion.

Sediment yield from the tectonically active semiarid Western Transverse Ranges of California

Sediment yields from the worlds rivers are generally highest from steep drainage basins with weak lithology, active tectonics, or severe land-use impacts. Here, we evaluate sediment yields from the Western Transverse Ranges of California in an attempt to explain why they are two- to tenfold greater than the surrounding areas of California. We found that suspended-sediment yields across the gauged basins of the Western Transverse Range during 1969–1999 varied by approximately an order of magnitude (740–5300 t/km2/yr). Similarly, fine-sediment concentrations for normalized discharge rates varied by almost two orders of magnitude (e.g., 1.3–110 g/L for the mean annual flood) for 11 previously unmonitored drainages of the Santa Ynez Mountains. Areas with high sediment yields consistently have weakly consolidated bedrock (Quaternary-Pliocene marine formations) and are associated with the highest rates of tectonic uplift of the region (>5 mm/yr). These regions are important to the sediment discharge budgets, because ~50% of the total suspended-sediment discharge from the Western Transverse Range is estimated to be generated within these regions, even though they represent only ~10% of the total watershed area. Previous estimates of suspended-sediment discharge from the Ventura River have likely been underestimated by ~50% because the gauging station is located immediately upstream of a high sediment yield region. We also found a significant and positive correlation between sediment yield and the percentage of a watershed with grassland and agricultural land use. These results suggest that there is adequate variation within the lithology, tectonics, and land use of the broader Western Transverse Range geologic province to induce large variations in sediment yield at the local scale.

Geomorphic effectiveness, sandur development, and the pattern of landscape response during jökulhlaups: Skeiðarársandur, southeastern Iceland

By contrast with other historical outburst floods on SkeiTararsandur, the 1996 jokulhlaup was unprecedented in its magnitude and duration, attaining a peak discharge of f53,000 m 3 /s in <17 h. Using a combination of field sampling and remote sensing techniques (Landsat TM, SAR interferometry, airphotos, and laser altimetry), we document the sandur-wide geomorphic impacts of this event. These impacts varied widely across the SkeiTararsandur and cannot be singularly attributed to jokulhlaup magnitude because pre-jokulhlaup glacial dynamics and the extant setting largely conditioned the spatial pattern, type, and magnitude of these impacts. Topographic lowering and asymmetric retreat of the ice front during the late twentieth century has decoupled the ice sheet from the moraine/sandur complex along the central and western sandur. This glacial control, in combination with the convex topography of the proximal sandur, promoted a shift from a primarily diffuse-source braided outwash system to a more point-sourced, channelized discharge of water and sediment. Deposition dominated within the proglacial depression, with approximately 3.8*10 7 m 3 of sediment, and along channel systems that remained connected to subglacial sediment supplies. This shift to a laterally dissimilar, channelized routing system creates a more varied depositional pattern that is not explicitly controlled by the concave longitudinal profile down-sandur. Laterally contiguous units, therefore, may vary greatly in age and sediment character, suggesting that current facies models inadequately characterize sediment transfers when the ice front is decoupled from its sandur. Water was routed onto the sandur in a highly organized fashion; and this jokulhlaup generated major geomorphic changes, including sandur incision in normally aggradational distal settings and eradication of proximal glacial landforms dating to fA.D. 1892. D 2002 Elsevier Science B.V. All rights reserved.