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Charles A. Nittrouer

Publications and source records attributed to Charles A. Nittrouer.

5 recordsLinked to original sources
USGS2021

Amazon sediment transport and accumulation along the continuum of mixed fluvial and marine processes

Sediment transfer from land to ocean begins in coastal settings and, for large rivers such as the Amazon, has dramatic impacts over thousands of kilometers covering diverse environmental conditions. In the relatively natural Amazon tidal river, combinations of fluvial and marine processes transition toward the ocean, affecting the transport and accumulation of sediment in floodplains and tributary mouths. The enormous discharge of Amazon fresh water causes estuarine processes to occur on the continental shelf, where much sediment accumulation creates a large clinoform structure and where additional sediment accumulates along its shoreward boundary in tidal flats and mangrove forests. Some remaining Amazon sediment is transported beyond the region near the river mouth, and fluvial forces on it diminish. Numerous perturbations to Amazon sediment transport and accumulation occur naturally, but human actions will likely dominate future change and now is the time to document, understand, and mitigate their impacts.

Amazon↗
USGS2019

Seasonal, tidal, and geomorphic controls on sediment export to Amazon River tidal floodplains

Mainstem–floodplain material exchange in the tidal freshwater reach of ma jor rivers may lead to significant sequestration of riverine sediment, but this zone remains understudied compared to adjacent fluvial and marine environ ments. This knowledge gap prompts investigation of floodplain-incising tidalchannels found along the banks of tidal rivers and their role in facilitating water and suspended-sediment fluxes between mainstem and floodplain. To evaluate this role, and how it evolves along the tidal river and with time, we measured water level, flow velocity, temperature, and suspended-sediment concentration (SSC) in four tidal channels along the tidal Amazon River, Brazil. Eleven deployments were made during low, rising, high, and falling seasonal Amazon discharge. Generally, channels export high-SSC water from the mainstem to the tidal floodplain on flood tides and transfer low-SSC water back to the mainstem on ebbs. Along the length of the tidal river, the interaction between tidal and seasonal water-level variations and channel–floodplain morphology is a primary control on tidal-channel sediment dynamics. Close to the river mouth, where tides are large, this interaction produces transient flow features and current induced sediment resuspension, but the importance of these processes decreases with distance upstream. Although the magnitude of the exchange of water and sediment between mainstem and floodplain via tidal channels is a small percentage of the total mainstem discharge in this large tidal-river system, tidal channels are important conduits for material flux between these two environments. This flux is critical to resisting floodplain submergence during times of rising sea level.

Amazon River↗
USGS2008

Timing and patterns of basin infilling as documented in Lake Powell during a drought

Between 1999 and 2005, drought in the western United States led to a >44 m fall in the level of Lake Powell (Arizona-Utah), the nation's second-largest reservoir. River discharges to the reservoir were halved, yet the rivers still incised the tops of deltas left exposed along the rim of the reservoir by the lake-level fall. Erosion of the deltas enriched the rivers in sediment such that upon entering the reservoir they discharged plunging subaqueous gravity flows, one of which was imaged acoustically. Repeat bathymetric surveys of the reservoir show that the gravity flows overtopped rockfalls and formed small subaqueous fans, locally raising sediment accumulation rates 10–100-fold. The timing of deep-basin deposition differed regionally across the reservoir with respect to lake-level change. Total mass of sediment transferred from the lake perimeter to its bottom equates to ~22 yr of river input.

Arizona, Utah↗
USGS1999

Modern accumulation rates and a sediment budget for the Eel shelf: a flood-dominated depositional environment

The northern California continental margin is periodically impacted by geologically significant storms, which have a marked influence on terrigenous sediment supply, flood deposition, and long-term accumulation of fine-grained sediment on the Eel shelf. Accumulation of Eel River muds on the adjacent shelf was investigated using 210Pb and 137Cs geochronologies, in order to understand the fate of sediment discharged by the Eel River and to relate patterns of net sediment accumulation (100-yr time scale) to sediment dynamics. 210Pb data demonstrate that modern accumulation of river mud occurs from the 50-m isobath seaward. Across-shelf accumulation rates decrease from maximum mid-shelf values of 0.6–1.7 g cm2 yr1 to values of 0.2–0.4 g cm2 yr1 at the shelf break, with a spatially weighted mean of 0.5 g cm2 yr1 (0.4 cm=yr) for the entire shelf. 210Pbxs sediment-depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity, which are produced by flood deposition. In some cores, particular 210Pbxs activity intervals may be associated with major Eel River floods of 1955, 1964, and 1974. It is postulated that, because of the coincidence of high-river-flow events and southerly winds during cyclonic winter storms, net northward transport allows for preferential deposition of fine-grained sediment north of the river mouth. Over the past ¾100 years, fluvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter, evident by both the spatial distribution of 210Pb accumulation rates and by clay-rich flood layers partially preserved in shelf deposits. A fine-grained sediment budget for the dispersal system, based on hydrological data and 210Pb geochronologies, demonstrates that a maximum of ¾20% (3 ð 109 kg=yr) of the mean annual supply of fluvial mud (14 ð 109 kg=yr) is trapped on the shelf. The results of this study demonstrate that: (1) short-term sedimentation processes associated with floods can influence sediment accumulation on longer time scales; and (2) a major fraction of fine-grained sediment supplied to tectonically active margins by flood-prone mountainous rivers bypasses narrow continental shelves.

Marine Geology↗