Gordon E. Grant

Moving beyond heterogeneity and process complexity: A new vision for watershed hydrology

Field studies in watershed hydrology continue to characterize and catalogue the enormous heterogeneity and complexity of rainfall runoff processes in more and more watersheds, in different hydroclimatic regimes, and at different scales. Nevertheless, the ability to generalize these findings to ungauged regions remains out of reach. In spite of their apparent physical basis and complexity, the current generation of detailed models is process weak. Their representations of the internal states and process dynamics are still at odds with many experimental findings. In order to make continued progress in watershed hydrology and to bring greater coherence to the science, we need to move beyond the status quo of having to explicitly characterize or prescribe landscape heterogeneity in our (highly calibrated) models and in this way reproduce process complexity and instead explore the set of organizing principles that might underlie the heterogeneity and complexity. This commentary addresses a number of related new avenues for research in watershed science, including the use of comparative analysis, classification, optimality principles, and network theory, all with the intent of defining, understanding, and predicting watershed function and enunciating important watershed functional traits.

Peak flow responses to clear-cutting and roads in small and large basins, western Cascades, Oregon

This study quantified long-term changes in streamflows associated with clear- cutting and road construction and examined alternative hydrologic mechanisms to explain stream hydrograph changes in the Cascades Range, western Oregon. We examined differences in paired peak discharges for 150 to 375 storm events for five basin pairs, using 34-year records from two pairs of 60-to-101-ha experimental basins in the H. J. Andrews Experimental Forest, and 50-to-55-year records from three pairs of adjacent basins ranging from 60 to 600 km2. Forest harvesting has increased peak discharges by as much as 50% in small basins and 100% in large basins over the past 50 years. These increases are attributable to changes both in flow routing due to roads and in water balance due to treatment effects and vegetation succession.

Pattern and origin of stepped-bed morphology in high-gradient streams, Western Cascades, Oregon

A general hierarchical framework for viewing stepped-bed morphology in high-gradient channels is presented. We emphasize channel units—bed features that are one or more channel widths in length—as a particularly important scale of variation. Field studies in two streams in the Cascade Range in Oregon indicated that pool, riffle, rapid, cascade, and step channel units had distinct bed slope ranges, with average slopes of 0.005, 0.011, 0.029, 0.055, and 0.173, respectively. Steeper units (rapids and cascades) are composed of step-pool sequences created by particles representing the 90th or larger percentile size fraction of bed material. Step spacing is inversely proportional to bed slope. The distribution of channel units along a stream is influenced by bedrock and processes that introduce coarse sediment. Cascade and pool units dominate where landslide and debris-flow deposits constrict channel width and deliver large immobile boulders to the channel, whereas riffle and rapid units dominate in broad valley flats where deposition of finer sediment occurs. Markov chain analysis indicates that channel units occur in nonrandom two-unit sequences with the slope of the upstream unit inversely proportional to the slope of the next downstream unit. Pool-to-pool spacings average two to four channel widths, but variability in spacing is high, owing to uneven distribution of bedrock out-crops and boulder deposits within the channel. Hydraulic reconstruction indicates that channel units form during high- magnitude, low-frequency events with recurrence intervals of about 50 yr. Comparison of channel-unit morphology to high-gradient flume experiments with heterogenous bedload mixtures indicated that unit morphogenesis is linked to factors that cause congestion of large particles during bedload transport events; these include local constrictions in channel width, immobile bed material, and abrupt fluctuations in velocity due to hydraulic jumps that promote deposition. Channel units appear to be a two-dimensional bar form found in streams where gradients exceed 2%, bedload is widely sorted, and width-to-depth ratios and sediment supply are low—conditions found in many mountain environments.

When do logs move in rivers

Large woody debris is an integral component of forested, fluvial systems throughout the world, yet we know little about hydraulic thresholds for movement and transport of logs. We developed theoretical models of entrainment and performed flume experiments to examine thresholds for wood movement in streams. Both the model and the experiments indicate that log entrainment is primarily a function of the piece angle relative to flow direction, whether or not the log had a rootwad, the density of the log, and the piece diameter. Stability increased if the pieces had rootwads or were rotated parallel to flow. Although previously reported as the most important factor in piece stability, piece length did not significantly affect the threshold of movement in our experiments or our physically based model, for logs shorter than channel width. These physically based models offer a first-order approach to evaluating the stability of either naturally derived woody debris or material deliberately introduced to streams for various management objectives.

Dynamics of Wood Transport in Streams: A Flume Experiment

The influence of woody debris on channel morphology and aquatic habitat has been recognized for many years. Unlike sediment, however, little is known about how wood moves through river systems. We examined some dynamics of wood transport in streams through a series of flume experiments and observed three distinct wood transport regimes: uncongested, congested and semi-congested. During uncongested transport, logs move without piece-to-piece interactions and generally occupy less than 10 per cent of the channel area. In congested transport, the logs move together as a single mass and occupy more than 33 per cent of the channel area. Semi-congested transport is intermediate between these two transport regimes. The type of transport regime was most sensitive to changes in a dimensionless input rate, defined as the ratio of log volume delivered to the channel per second (Qlog) to discharge (QW); this ratio varied between 0·015 for uncongested transport and 0·20 for congested transport. Depositional fabrics within stable log jams varied by transport type, with deposits derived from uncongested and semi-congested transport regimes having a higher proportion of pieces orientated normal to flow than those from congested transport. Because wood input rates are higher and channel dimensions decrease relative to piece size in low-order channels, we expect congested transport will be more common in low-order streams while uncongested transport will dominate higher-order streams. Single flotation models can be used to model the stability of individual pieces, especially in higher-order channels, but are insufficient for modelling the more complex intractions that occur in lower-order streams.

Transport and deposition of large woody debris in streams: a flume experiment

Abstract Large woody debris (LWD) is an integral component of forested streams of the Pacific Northwest and elsewhere, yet little is known about how far wood is transported and where it is deposited in streams. In this paper, we report the results of flume experiments that examine interactions among hydraulics, channel geometry, transport distance and deposition of floating wood. These experiments were carried out in a 1.22-m-wide×9.14-m-long gravel bed flume using wooden dowels of various sizes as surrogate logs. Channel planforms were either self-formed or created by hand, and ranged from meanders to alternate bars. Floating pieces tended to orient with long axes parallel to flow in the center of the channel. Pieces were deposited where channel depth was less than buoyant depth, typically at the head of mid-channel bars, in shallow zones where flow expanded, and on the outside of bends. We hypothesize that the distance logs travel may be a function of the channels debris roughness, a dimensionless index incorporating ratios of piece length and diameter to channel width, depth and sinuosity. Travel distance decreased as the ratio of piece length to both channel width and radius of curvature increased, but the relative importance of these variables changed with channel planform. Large pieces can move further than our debris roughness models predict if greater than 50% of the active channel area is deeper than the buoyant depth of the piece, or if momentum is high enough to carry pieces across shallows. Our debris roughness model allows first-order prediction of the amount of wood transport under various channel geometries.

A Geological Framework for Interpreting Downstream Effects of Dams on Rivers

Despite decades of research and abundant case studies on downstream effects of dams on rivers, we have few general models predicting how any particular river is likely to adjust following impoundment. Here we present a conceptual and analytical framework for predicting geomorphic response of rivers to dams, emphasizing the role of geologic setting and history as first-order controls on the trajectory of change. Basin geology influences watershed and channel processes through a hierarchical set of linkages, extending from the drainage basin to the valley and channel, which determine the sediment transport and discharge regimes. Geology also directly shapes the suite of hillslope processes, landforms, and geomorphic disturbances impinging on the channel and valley floor. These factors, in turn, affect the “lability” or capacity for adjustment of the downstream channel, determining the type, direction, and extent of channel adjustments that occur, including incision, widening, and textural changes. We develop an analytical framework based on two dimensionless variables that predicts geomorphic responses to dams depending on the ratio of sediment supply below to that above the dam (S*) and the fractional change in frequency of sediment-transporting flows (T*). Drawing on examples from the Green, Colorado, and Deschutes Rivers, we explore how trajectories of geomorphic change, as defined by these two variables, are influenced by the geological setting and history of the river. This approach holds promise for predicting the magnitude and trend of downstream response to other dammed rivers, and can identify river systems where geological controls are likely to dominate.

Riparian forest disturbances by a mountain flood - the influence of floated wood.

Large floods can have major impacts on riparian forests. Here we examine the variability and spatial distribution of riparian forest responses along eight third- to fifth-order streams following a large flood (∼100 year recurrence interval) in the Cascade Mountain Range of Oregon. We categorized disturbance intensity (physical force) exerted on riparian trees during floods into three classes: (i) purely fluvial (high water flow only); (ii) fluvial supplemented by dispersed pieces of floating wood (uncongested wood transport); (iii) fluvial with movement of batches of wood (congested wood transport). These types of material transport and associated classes of disturbance intensity resulted in a gradient of biotic responses of disturbance severity ranging from standing riparian trees inundated by high water, to trees toppled but still partially rooted, to complete removal of trees. High within-stream and among-stream responses were influenced by pre-flood stream and riparian conditions as well as flood dynamics, especially the availability of individual pieces or congested batches of wood. Fluvial disturbance alone toppled fewer riparian trees than in reaches where floodwaters transported substantial amounts of wood. Debris flows delivered additional wood and sediment to parts of reaches of four of these study streams; riparian trees were removed and toppled for up to 1·5 km downstream of the debris flow tributary channel. Congested wood transport resulted in higher frequency of toppled trees and greater deposition of new wood levees along channel margins. The condition of the landscape at the time of a major flood strongly influenced responses of riparian forests. Recent and historic land-use practices, as well as the time since the previous large flood, influenced not only the structure and age of the riparian forests, but also the availability of agents of disturbance, such as large pieces of floating wood, that contribute to disturbance of riparian forests during floods. Copyright

1000 dams down and counting

Dam removals are reconnecting rivers in the United States Forty years ago, the demolition of large dams was mostly fiction, notably plotted in Edward Abbeys novel The Monkey Wrench Gang. Its 1975 publication roughly coincided with the end of large-dam construction in the United States. Since then, dams have been taken down in increasing numbers as they have filled with sediment, become unsafe or inefficient, or otherwise outlived their usefulness (1) (see the figure, panel A). Last years removals of the 64-m-high Glines Canyon Dam and the 32-m-high Elwha Dam in northwestern Washington State were among the largest yet, releasing over 10 million cubic meters of stored sediment. Published studies conducted in conjunction with about 100 U.S. dam removals and at least 26 removals outside the United States are now providing detailed insights into how rivers respond (2, 3).

Watering the forest for the trees: an emerging priority for managing water in forest landscapes

Widespread threats to forests resulting from drought stress are prompting a re-evaluation of priorities for water management on forest lands. In contrast to the widely held view that forest management should emphasize providing water for downstream uses, we argue that maintaining forest health in the context of a changing climate may require focusing on the forests themselves and on strategies to reduce their vulnerability to increasing water stress. Management strategies would need to be tailored to specific landscapes but could include thinning, planting and selecting for drought-tolerant species, irrigating, and making more water available to plants for transpiration. Hydrologic modeling reveals that specific management actions could reduce tree mortality due to drought stress. Adopting water conservation for vegetation as a priority for managing water on forested lands would represent a fundamental change in perspective and potentially involve trade-offs with other downstream uses of water.