Richard D. Woodsmith

MULTIVARIATE GEOMORPHIC ANALYSIS OF FOREST STREAMS: IMPLICATIONS FOR ASSESSMENT OF LAND USE IMPACTS ON CHANNEL CONDITION

Multivariate statistical analyses of geomorphic variables from 23 forest stream reaches in southeast Alaska result in successful discrimination between pristine streams and those disturbed by land management, specifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model discriminates 10 disturbed from 13 undisturbed reaches with 90 per cent and 92 per cent correct classification respectively. These variables are the total number of pools per reach, the ratio of mean residual pool depth to mean bankfull depth, and the ratio of critical shear stress of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classification; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, can also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. However, channel unit classification and inventory can be subject to considerable error and observer bias. Abundance of pool-related large woody debris is highly correlated with pool frequency and is an important factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and disturbed channel conditions, providing a reference standard for channel assessment and restoration efforts.

Characteristics of channel steps and reach morphology in headwater streams, southeast Alaska

Abstract The effect of timber harvesting and mass movement on channel steps and reach morphology was examined in 16 headwater streams of SE Alaska. Channel steps formed by woody debris and boulders are significant channel units in headwater streams. Numbers, intervals, and heights of steps did not differ among management and disturbance regimes. A negative exponential relationship between channel gradient and mean length of step intervals was observed in the fluvial reaches (

Changes in hyporheic exchange flow following experimental wood removal in a small, low-gradient stream

[1] We investigated the response of hyporheic exchange flow (HEF) to wood removal in a small, low-gradient, gravel bed stream in southeast Alaska using a series of groundwater models built to simulate HEF for the initial conditions immediately after wood removal and 1 month, 2 years, 4 years, and 16 years following wood removal. The models were based on topographic surveys of the stream channel and surrounding floodplain, and surveyed water surface elevations (WSEs) were used to assign stream boundary conditions. Using the groundwater flow model, MODFLOW, and the particle tracking model, MODPATH, we calculated hyporheic exchange fluxes, their residence time distributions, and both longitudinal and plan view spatial patterns of downwelling and upwelling zones. In the first few years, streambed scour and sediment deposition smoothed the streambed and WSE profile, reducing HEF. Also, large contiguous patches of downwelling or upwelling were fragmented, nearly doubling the total number of patches present on the streambed. As the stream continued to adjust to the loss of wood, those trends began to reverse. Accretion of sediment onto alternating bars resulted in better developed pool-riffle morphology, enhanced HEF, and increased residence times and also resulted in downwelling and upwelling zones coalescing into elongated patches along bar margins. This study showed that the hyporheic zone is sensitive to changes in wood loading and that initial changes in HEF resulting from the direct effects of wood removal were contrary to longer-term channel adjustments to changes in wood loading.

Chapter 10 Maintenance of an obstruction-forced pool in a gravel-bed channel: streamflow, channel morphology, and sediment transport

Maintenance of pool morphology in a stream channel with a mobile bed requires hydraulic conditions at moderate to high flows that route bed load through the pool as it is delivered from upstream. Through field measurements of discharge, vertical velocity profiles, bed load transport, and streambed scour, fill, and grain-size distribution, we found that maintenance of a pool associated with a large, in-channel obstruction was more adequately explained by analogy to scour processes at bridge abutments than by the concept of velocity reversal. The cross-sectional area through the deepest part of the pool remained remarkably stable throughout a wide range in discharge magnitude. This occurred despite transport of significant quantities of bed load from upstream through the pool, well above and well below bankfull discharge and on rising as well as falling hydrograph limbs. There was no tendency for near-bed velocity or shear stress (averaged over the cross-sectional width of active bed load transport) at the pool centre to exceed that at the upstream riffle tail (pool head) or downstream riffle head (pool tail) as discharge increased up to at least 1.3 times bankfull. Fractional rates indicated that at the pool centre significant bed load transport was initiated and approached 100 g m−1 s−1 at notably lower mean bed shear stress than at the pool head or tail. Furthermore, incipient motion analyses suggested that mean bed shear stress entrainment thresholds were lower at the pool centre than at the pool head or tail. These findings indicated that total entrainment force in the pool was underestimated by average bed shear stress alone. Through inference, rather than direct measurement, we concluded that, as at bridge abutments, turbulent effects generated by interaction of streamflow with the obstruction added a component of total entrainment force. We further inferred that this combination of mean bed shear stress and instantaneous turbulent force was responsible for entrainment and transport of sediment, thereby maintaining pool morphology. This conceptual model of pool maintenance through a combination of mean bed shear stress and large-scale turbulent force suggests that flow obstructions in gravel-bed streams may be a dominatt factor, perhaps as important as mean hydraulic variables or caliber and volume of sediment supply, in controlling local channel morphology and local bed load dynamics.