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Abstract

Paleohydraulic data from outwash terrace sequences in New Zealand and Iceland suggest that they were formed by glacially controlled, high magnitude, incising flood events. Relic incision channels cut into the terrace surfaces provide evidence of steeper gradients, coarser sediment and thus greater bed-shear stresses, velocities, Froude numbers, and discharges than those of the braided channel systems on the terrace surfaces.

The paper proposes that channel incision and terrace formation can be generated by catastrophic floods which exceed the thresholds for channel entrenchment. These thresholds are defined by the flow conditions predicted by paleohydraulic modeling of the relic channels.

Abstract

Stratification and estimated paleoflow conditions for valley-fill deposits suggest that Ogallala Group streams in western Nebraska were similar to modern streams of south-central Alberta.

Ogallala stratification includes medium-scale (0.5 to 2.0 m thick) trough crossbedded sand and gravel, tabular indistinctly horizontally bedded and imbricated gravel, and horizontally bedded sand and pebbly sand. Valley fills are 15 to 55 m thick and 800 to 1800 m wide at the top. Some are in bedrock-floored channels resembling the “inner channels “ of Shepherd and Schumm (1974).

Gradients for three well exposed paleovalley floors range from 0.0014 to 0.0020 (m/m) after tectonic correction. This compares with 0.00135 for the modern North Platte River Valley in Nebraska. The average intermediate diameters of the 10 largest clasts from tabular gravel beds found at 17 sites varied between 0.077 and 0.15 m. The average median intermediate diameter for gravel from four well exposed tabular gravel beds is 0.024 m.

Consistent paleodepth estimates of about 2 m correlate well with the scale of cross-stratification observed in the valley fills. Paleo-velocities are estimated at about 2 m/sec, and Froude numbers of about 0.4 are consistent with a lower flow regime in the stability field of dunes. Two-dimensional specific in-channel paleodischarges were 3 to 4 m2/sec. Total paleodischarge estimates based on slope-discharge relationships for gravel-bed rivers range from 340 to 1240 m3/sec and are comparable to average annual peak discharges on the North Platte River reported 80 to 90 years ago.

Ogallala streams were probably dominated by macroforms similar to the “crescent-shaped bars” of the North Saskatchewan River. Deposition also took place on longitudinal bars in deeper channels. Shallow upper-flow regime transport and deposition is recorded by horizontally bedded sand and pebbly sand.

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