Influences on Bed Sorting and Armoring in an Upland Gravel-Cobble Bed River, Middle Fork John Day River, Oregon

Abstract:

The Middle Fork is an unconfined to partly confined upland river with channel length of 34 km, drainage area of 250-850 km², and channel gradients of 0.004 to 0.006 in the study area. Geology is dominated by Tertiary volcanic and volcaniclastic rocks that yield abundant coarse clasts. Surface and subsurface bed material was sampled volumetrically at twenty-five sites. The textural types range from gravelly cobbles to sandy cobbly gravels, sand content is low (2 to 13%), mud content is very low, and sorting is poor to very poor.

Generally the surface layer is an open framework gravel or cobble, while the subsurface layer is a filled or partially filled framework gravel or cobble. Despite an armored appearance, only 20% of the sites are armored using the standard armor ratio (surface D₅₀/subsurface D₅₀). While surface layers are not coarser than their subsurface layers in terms of the median or coarse end of the distribution, they are coarser in terms of fines (ratios based on D₂₅, D₁₆, % sand), suggesting that alternatives to the D₅₀armor ratio might be useful.

Multivariate analysis of size fraction data reveals four distinct groups of samples, distinguished mainly by differences in proportions of coarse to fine gravels, and in abundance of sand. While one group comprises only surface samples and another subsurface samples, two of the groups are mixed.

One goal of the project is to evaluate the effects of land use history on bed material characteristics and mobility. Sediment characteristics were examined in relation to distance downstream, geology, relation to debris-flow sources, land use history, and other potential influences. There are no geologic associations or downstream trends in fining or other grain size parameters. Differences in land use history, such as former dredged-mined reaches and reaches with recent restoration projects also do not explain patterns of armoring or other sediment characteristics. High variability within each reach suggests that local hydraulic factors may be influencing sediment characteristics.