Geomorphology and sedimentology of the Holocene Slave River delta, Northwest Territories

Bibliography: p. 65-68.The Slave River Delta (8,300 km2) is a long (170 km), narrow ( 42 km) alluvial plain which extends north from the Slave River Rapids at Fort Smith to the Great Slave Lake. The delta plain is flanked by the Talston River, Tethul River and Canadian Shield to the east and Little Buffalo River to the west. Analysis of 36 litho-stratigraphic logs from river cutbanks indicate a sandy wave-influenced delta inferred from the dominance of wave-associated sedimentary structures in the middle, upper shoreface and beach deposits. The cutbank exposures terminate approximately 235 km downriver from Fort Smith where mud dominates much of the surficial sediment. Receiving basin morphology, water depth and termination of rebound in the region appears to be accountable for the transition. Radiocarbon analysis of 11 wood samples from river cutbanks and a paleoshoreline reconstruction indicate that the delta prograded at an average rate of 20.76 metres per year from 8,070 to the present. A tilt rate of 21.2 cm/km due to isostatic rebound, normal to the retreating ice margin, has been calculated for the Slave Delta region. The subaqueous delta front exhibits several unique morphologic features including barrier islands, offshore bars, tensional cracks, subaqueous slumps and pressure ridges at 59 m lake depth. The barriers and off shore bars consist of medium sand while the slumps and pressure ridges are interpreted to be of mud

Late-Holocene Shoreline Responses to Competing Shelf, Bay, and Beach Accommodation Spaces Under Conditions of Relative Sea Level Change, and the Potential for Future Catastrophic Beach Retreat in the Columbia River Littoral Cell, Washington and Oregon, USA

The Columbia River Littoral Cell (CRLC) (160 km in length) provides opportunities to compare competing accommodation space relations under different conditions of relative sea level change. The CRLC system includes abundant littoral sand supply from the large Columbia River, late-Holocene prograded beach plains and barrier spits (0.5–5 km in width), two large marine-dominated estuaries (Willapa Bay and Grays Harbor), and a high-wave-energy inner-shelf. Littoral sand accumulation rates in prograded beach plains and barrier deposits are based on paleo-shoreline positions that are dated by great-earthquake catastrophic beach retreat scarps (n = 10) from 0.3 to 5.0 ka. The retreat scarp timelines are mapped in across-shore GPR transects (n = 79), yielding timeline-bounded polygons (n = 247). The polygons are evaluated for littoral sand volume and bounding ages, yielding volume accretion rates (m3 ka−1), which are summed for the four CRLC subcells; Clatsop Plains (27.4 × 106 m3 ka−1), Long Beach (19.8 × 106 m3 ka−1), Grayland Plains (15.9 × 106 m3 ka−1), North Beaches (11.7 × 106 m3 ka−1). Major submarine sinks of littoral sand, including the inner-shelf and large marine-dominated estuaries, are evaluated for increased littoral sand accommodation space that could result from potential future sea level rise of 1, 2 and 3 m during the next century or two. The estimated beach and nearshore sand erosion needed to fill the increased submarine accommodation space from a 2.0 m rise in relative sea level would result in averaged beach retreat values of ~0.7 km (Clatsop Plains), ~1.2 km (Long Beach), and ~ 1.3 km (Grayland Plains), about 30–50% larger than previous estimates based on the Brunn method. These catastrophic shoreline retreat distances (0.7–1.3 km) represent 25–50% of the present widths of prograded barrier spits and beach plains. They serve as warnings about future catastrophic beach erosion resulting from potential future SLR in other similar barrier spit and beach plain shorelines worldwide

Catastrophic Beach Sand Losses Due to Erosion from Predicted Future Sea Level Rise (0.5–1.0 m), Based on Increasing Submarine Accommodation Spaces in the High-Wave-Energy Coast of the Pacific Northwest, Washington, Oregon, and Northern California, USA

The U.S. Pacific Northwest (PNW) coastline (1000 km) has been analyzed for conditions that could impact beach erosion from potential near-future (100 year) sea level rise (SLR). Heavy mineral analysis of river, beach, and shelf samples (n = 105) establish the sources of the beach deposits. River bedload discharge and intervening estuarine sinks for river sand supplies (n = 31) were normalized to the one century time interval. Twenty-six subcell beaches (657 km in combined length) were surveyed (153 profiles) for beach sand widths (20–412 m) and sand cross-sectional areas (20–1810 m2 ) above wave-cut platforms and/or 0 m tidal datum. Cross-sectional areas were multiplied by beach segments to yield subcell beach sand volumes (0.4 × 106 m3 –35.8 × 106 m3 ± 20% uncertainty). Innermost-shelf profiles were measured for distance to the 100-year depth of closure (30 m) to digitize the areas of inner-shelf accommodation space. Both innermost-shelf and estuarine accommodation space volumes for beach sand displacements were established for 0.5 and 1.0 m SLR. The existing subcell beach sand volumes and computed new beach sand supplies (rivers and longshore transport) were subtracted from the estimated sand volumes lost to submarine accommodation spaces to establish potential beach sand deficits from near-future SLR. Of the 26 surveyed active-beaches, some 60% and 80% (by length) are predicted to be lost, respectively, from the 0.5 m and 1.0 m SLR or equivalent littoral sand sedimentation in submarine accommodation spaces. Projected losses reach 90% for all PNW beaches (~900 km total length) from 1.0 m SLR. The computed beach sand deficits are used to estimate soft-sand retreat distances or erosional beach step backs (50–590 m ± 35% uncertainty) in unrevetted barrier spit and beach/dune deflation plains from 1.0 m SLR. Such empirical accommodation space analyses should have worldwide relevance to predicting beach erosion from near-future SLR

Late Holocene Chronology and Geomorphic Development of Fluvial-Tidal Floodplains in the Upper Reaches of the Lower Columbia River Valley, Washington and Oregon, USA

The upper reaches of the lower Columbia River Valley (125km in length) comprise an alluvial system that is transitional between fluvial and fluvial-tidal dominance. Sinuous channels separate elongate islands (1-8km in length) and floodplains (0.5-12.7km in total width). Thirty-six floodplain overbank deposits are analyzed for age and depth, which demonstrate an average sedimentation rate of 1.6mka-1 during the last 5-6ka. Older core records confirm that long-term depositional rates are controlled by relative sea level rise. Rising floodplain groundwater surfaces, which followed relative sea level rise (~1.25mka-1), submerged isolated floodplain depressions. Low sedimentation rates in the isolated depressions (0.6-1.1mka-1) maintained large ellipsoidal bullseye lakes (7-22km2 in area) dating back to 3.5-4.0ka. Increases in the widths of the floodplains and bullseye lakes are associated with broadening of the incised valley (4-13km width) in the Portland Basin. Dated basal overbank deposits (0.5-5.0ka in age) and their separation distances establish channel migration rates of 0.3-1.9kmka-1. Shallow burial rates relative to rapid channel migration rates resulted in reworking of late Holocene floodplains (50-75% erosion) since 5ka in the upper reaches of the lower Columbia River Valley