Steep Headwater Streams Research Articles

Gas Transfer Velocities Evaluated Using Carbon Dioxide as a Tracer Show High Streamflow to Be a Major Driver of Total CO2 Evasion Flux for a Headwater Stream

AbstractEvasion of carbon dioxide (CO2) from headwater streams is a dominant process controlling the fate of terrestrially derived carbon in inland waters. However, limitations of sampling techniques inhibit efforts to accurately characterize CO2 evasion from streams, and particularly headwater streams with steep gradients, complex morphologies, and challenging terrain. CO2 source dynamics coupled with turbulence conditions control gas transfer velocities of CO2 ( ) and therefore drive CO2 evasion. We present estimates of and CO2 evasion from a steep, turbulent headwater stream in southwestern British Columbia, Canada, collected using an automated in situ CO2 tracer technique. Gas transfer velocities scaled positively with discharge, with a median of 36.8 m/day and a range of 13.5 to 169 m/day. Gas transfer velocities were highest during high‐flow events, with 84% of all CO2 emissions occurring when discharge was higher than Q50, the median discharge (92.6 L/s). Widely used models overestimated gas transfer velocities with a mean relative error of 24% but underestimated k600 values above 165 m/day. Our determinations of gas transfer velocities for a range of streamflow suggest that CO2 evasion may be higher than previously estimated from direct measurements or models, particularly during high‐flow events. These findings illustrate the need for direct, frequent, in situ determinations of to accurately characterize CO2 evasion dynamics in steep headwater streams.

Controls on log step occurrence in steep headwater streams draining Carpathian managed forests

Instream wood in steep headwater streams impacts channel morphology and sediment fluxes via the development of log steps and related sediment storage. I investigated detailed characteristics of 73 log steps in managed temperate Carpathian forests and assessed channel- and wood-based thresholds for log step development. The limited recruitment of wood from managed forests was apparent in the low frequency of log steps (an average of 1.3 steps per 100 m channel length) in bedload-dominated channels when compared to temperate and boreal old-growth forests, and an even lower number of log steps was observed in streams recently affected by debris flows or hyperconcentrated flows. The upstream threshold of log step occurrence can be assigned to those locations in stream longitudinal profiles, where transport capacity during ordinary high flows exceeds the critical value for the initiation of bedload movement. Downstream thresholds are controlled by stream transport capacity and wood dimensions, which correspond to the limited recruitment of large wood pieces in managed forests. All log steps were found in contributing basin areas smaller than 1 km2, and the majority of log steps (93.1%) originated in channels less than two metres in width. The development of a log step at a particular channel width or basin area was controlled by the wood length, but the resultant step parameters (step height and width, length of upstream sedimentary wedge and volume of stored sediments) were better correlated with wood diameter. The parameters of horizontal and vertical wood orientation had less importance in the development of a log step or its dimensions. Wood pieces shorter than 4 m in length accounted for 72.2% of the step-forming pieces, and the maximal diameter was less than 0.1 m for 20.5% of the pieces. These findings show the significance of relatively small wood pieces in log step development and sediment routing in steep headwater streams; the average sediment storage generated by a log step was 0.51 m3. The support of log step development and their preservation in headwater streams draining managed forests represents an alternative low-cost approach for sediment management at the basin scale.

Large wood potential, piece characteristics, and flood effects in Swiss mountain streams

We present results of two studies on the (1) potential wood load in steep headwater streams and (2) properties of large wood (LW) transported in mountain rivers during the large August 2005 flood event in Switzerland. Ten headwater reaches of 1000 m length were surveyed in different regions of Switzerland. The potential wood load was estimated for in-channel deadwood, and possible driving factors were explored. Correlations were found with dead wood volume on hillslopes and mean channel width. We established size distributions of LW pieces and identified probable recruitment processes. Four reaches were resurveyed after an exceptionally severe flood in August 2005, showing limited LW transport in channels but considerable wood input by mass wasting processes. In addition, characteristics of deposits of LW along mountain rivers affected by the 2005 flood were investigated. Diameter and length distribution of transported and deposited pieces were comparable to those of LW from steep headwater streams, yet with considerably fewer long pieces in the deposits of mountain rivers. Most LW pieces were fresh wood, indicating that the portion of in-channel deadwood transported during the 2005 flood was limited. Findings of the study contribute to a better understanding of LW dynamics in Alpine mountain streams.

Bedload transport and flow resistance in steep channels – introduction to the issues in the context of mountain basins of the Central European region

Although sediment transport in the uppermost parts of mountain basins may be supply- limited, bedload transport in small torrents represents a potential hazard during extraordinary floods. The authors attempt to review recent worldwide research of bedload and hydraulics the results of which could potentially be applied in local steep channels. Approaches to the calculation of flow resistance (Manning relationship, logarithmic and power-law relationships) and critical conditions of the incipient motion of a bed particle (critical shear stress, stream power, flow velocity and discharge) are closely discussed in order to describe hydraulic and bedload transport conditions in steep headwater streams. Difficulties are pointed out in connection with the estimation of critical conditions of bedload transport in headwater streams with poorly sorted bed material and more or less developed bedforms. In the second part of the paper the authors present two pilot studies from the Western Carpathians and test potential applicability of specific formulas in this region. Results of the formulas point to relatively nonselective character of bedload transport during high magnitude floods in Beskydian headwater streams. An important role is played by the intensity of sediment delivery to active channels in limited sediment-supply conditions. The authors also point out that a majority of bedload transport and flow resistance equations were originally created on the basis of flume experiments or they were tested in Alpine torrents and therefore it is supposed that they might work well in local torrents after being calibrated to local conditions.

Biogeomorphological effects of leaf accumulations in stepped-bed channels: Exploratory study, Moravskoslezské Beskydy Mountains, Czech Republic

Abstract The stepped-bed system, with a step-like longitudinal profile, is typical morphology in steep headwater streams. These systems are created by a series of coarse sediments or instream wood (steps with supercritical flows) interspaced with finer material (forming pools with subcritical flows). In the case of well-developed steps and pools, the resulting channel-reach morphology is referred to as “step-pool” morphology. In this study, we identify a previously undescribed type of step-pool formation, the “foliaged step-pool”, in the high-gradient Stoligy Stream of the Moravskoslezské Beskydy Mountains. The defining feature of this formation is the significant presence of leaves in the step structure. The geometry of the steps and pools was measured and the parameters that characterise the distribution, amount and function of leaves acting in these areas were defined. Statistical results showed differences between non-foliaged and foliaged step-pool formations, in which the latter showed a significant increase in storage level, influencing the channel’s hydrodynamics. Particle-size analyses demonstrated that foliaged step-pool formations had finer sediment in the pools, which indicates that there are differences in sediment transport processes between foliaged and non-foliaged formations. These results offer new insights into stepped-bed and step-pool morphology, providing directions for further research on small streams in deciduous forested regions.

Connectivity of the coarsest fraction in headwater channels: imprints of fluvial processes and debris‐flow activity

Some steep headwater streams of the mid‐mountains landscape of the flysch belt of the estern arpathians are affected by debris flows. Connectivity of the largest boulder and cobble fractions has been evaluated in steep streams found in the transitional zones between the hillslopes and alluvial cones of similar lithology and watershed morphometry. Two longitudinal profiles affected by past debris‐flow activity and two longitudinal profiles void of such processes were selected. In the first case, active channels were characterised by the presence of the coarsest fraction, while a downstream trend of sediment coarsening or fining lacked any influence of contemporary fluvial processes and sediment supply. In addition, debris‐flow‐affected streams showed greater differences between the middle axis of the coarsest particle and the particle‐size index including the lengths of all three axes, which is most likely the result of limited active fluvial transport. All the studied streams demonstrated general downstream fining of the largest particles, which resulted from local flysch lithology containing more resistant sandstones in the upper parts of the watershed. The trend of sediment fining was more rapid in the longitudinal profiles unaffected by past debris flows. These types of streams also showed adjustment of the coarsest bed fraction to contemporary processes in channels by sediment coarsening in incised reaches and sediment fining in depositional reaches as well as by downstream response to some lateral sediment inputs. The index of the unit stream power showed no correlation with the considered coarsest fraction in both fluvial‐dominated channels and debris‐flow‐affected channels.

Predictive equation for longitudinal dispersion coefficient

Dye tracing field data were collected in small, steep streams in Ontario and used to calculate longitudinal dispersion coefficients for these headwater streams. A predictive equation for longitudinal dispersion coefficient is developed using combined data sets from five steeper head – water streams and 24 milder and larger rivers. The predictive equation relates the longitudinal dispersion coefficient to hydraulic and geometric parameters of the stream and has been developed using multiple regression analysis. The newly developed equation shows impressive accuracy of predictions for longitudinal dispersion coefficient (R2 = 0.86, RMSE = 25, Nash–Sutcliffe coefficient Ens = 0.86 and Index of Agreement D = 0.96) for both small, steep headwater streams as well as large, mild rivers. The Froude number has been introduced as a third key parameter to capture the effect of slope of the reach – in addition to the aspect ratio and bed material surface roughness – on the longitudinal dispersion coefficient. The pronounced improvement in the accuracy of the prediction is due to the addition of the Froude number to capture the effect of the slope of the reach on longitudinal dispersion coefficient. Copyright © 2014 John Wiley & Sons, Ltd.

Bedload Transport and Morphological Effects of High-Magnitude Floods in Small Headwater Streams - Moravskoslezské Beskydy Mts. (Czech Republic)

Bedload transport observed during a flood in May 2010 gave rise to several forms of accumulations in small headwater basins located in the Western Flysch Carpathian Mountains, Czech Republic. We have investigated critical conditions of incipient motion of the largest boulders deposited during a c. Q100 flood event (flood competence method). We have tested several formulas designed for high gradient streams in two small basins in the conditions of local mid-mountain relief. The results show that a flood of such a magnitude is able to transport almost all surface bed material and that bedload transport in steep headwater streams (A ≤ 1 km 2 ) is probably less selective as for the grain size than that in lower gradient gravel-bed streams. The authors discuss the importance of local basin predispostion factors in order to determine critical conditions for the onset of bedload transport.

Controls on at‐a‐station hydraulic geometry in steep headwater streams, Colorado, USA

AbstractDetailed hydraulic measurements were made in nine step‐pool, five cascade and one plane‐bed reach in Fraser Experimental Forest, Colorado to better understand at‐a‐station hydraulic geometry (AHG) relations in these channel types. Average values for AHG exponents, m (0·49), f (0·39), and b (0·16), were well within the range found by other researchers working in steep gradient channels. A principal component analysis (PCA) was used to compare the combined variations in all three exponents against five potential control variables: wood, D84, grain‐size distribution (σ), coefficient of variation of pool volume, average roughness‐area (projected wetted area) and bed gradient. The gradient and average roughness‐area were found to be significantly related to the PCA axis scores, indicating that both driving and resisting forces influence the rates of change of velocity, depth and width with discharge. Further analysis of the exponents showed that reaches with m > b + f are most likely dominated by grain resistance and reaches below this value (m < b + f) are dominated by form resistance. Copyright © 2010 John Wiley & Sons, Ltd.

Disturbances structuring macroinvertebrate communities in steep headwater streams: relative importance of forest clearcutting and debris flow occurrence

This study shows that debris flow disturbances that alter in-channel physical conditions or displace organisms exert greater impacts on stream macroinvertebrate community structure than clearcutting disturbances that change energy inputs in steep headwater systems. We surveyed abiotic characteristics and macroinvertebrate communities of 10 steep headwater streams in central Japan that contained a chronosequence of forest stands and debris flow occurrences. Streams of recently logged forests had higher light levels, nitrate concentrations, and stream temperature ranges than streams of mature forests. Streams of middle-aged forests, which experienced debris flows in recent years (1989–1998), had lower abundance of shredders, crawlers, and headwater-adapted taxa than the other streams. A shredder taxon, Gammarus nipponensis , was completely absent in the streams with recent debris flows, despite their dominance in the other streams. Decreases of depositional environment associated with depletion of large wood and loss of channel structure as well as replacement of community members by rapid colonizers appear to be the major mechanisms of structural changes in macroinvertebrate communities affected by debris flows.

Sediment transfer processes in two Alpine catchments of contrasting morphological settings

Summary Coarse sediment transport processes in steep headwater streams may vary between bedload and debris flows events, depending on basin geomorphology and sediment supply. This paper compares two small catchments (drainage area 4–5 km 2 ) located in the Eastern Italian Alps where the dominant sediment transport processes differ substantially, i.e., debris flows in the Moscardo Torrent and bedload events in the Rio Cordon. The two basins were selected because they provided the unique opportunity to analyze long-term data of debris flow and bedload volumes, respectively. The two basins are compared in terms of peak discharge, event duration, and magnitude–frequency characteristics of transported sediment volumes. Additional bedload data from two more Alpine experimental basins are also considered. The results show that, for comparable recurrence intervals, debris flow volumes are 2–3 orders of magnitude larger than bedload volumes. This contrasting sediment transfer activity can be attributed to different basin and channel morphologies, which are analyzed in terms of sediment supply conditions, longitudinal profiles and slope–area curves.

Sediment and Wood Routing in Steep Headwater Streams: An Overview of Geomorphic Processes and their Topographic Signatures

Abstract Headwater streams in steep terrain pose a significant challenge for the development of best management practices (BMP) in forested watersheds. There is an incomplete understanding of the processes that govern the input, storage, and transport of sediment and wood, and these processes differ geographically. At the upstream extent of the channel network, headwater streams represent a transition from hillslope to channel processes. At the downstream extent, many of these channels transition from mass wasting to fluvial process dominance. Large-scale sediment routing processes typically consist of debris flows, earth flows, and/or gully erosion. In the interval between episodic transport events, headwater streams can interrupt the delivery of sediment from hillslopes to larger river systems by storing large volumes of sediment and wood. Forest management guidelines typically identify headwater streams as occurring upstream of the distribution of fish; however, a topographically based designation of headwater streams could prove useful for identifying which erosional processes are dominant in a particular area. Broad-scale terrain analysis based on unique topographic signatures can be used to identify the spatial domain of different geomorphic landforms that govern headwater stream processes, how they may be affected by forest management, and to infer the type and severity of downstream disturbance.

Processes and rates of sediment and wood accumulation in headwater streams of the Oregon Coast Range, USA

AbstractChannels that have been scoured to bedrock by debris flows provide unique opportunities to calculate the rate of sediment and wood accumulation in low‐order streams, to understand the temporal succession of channel morphology following disturbance, and to make inferences about processes associated with input and transport of sediment. Dendrochronology was used to estimate the time since the previous debris flow and the time since the last stand‐replacement fire in unlogged basins in the central Coast Range of Oregon. Debris flow activity increased 42 per cent above the background rate in the decades immediately following the last wildfire. Changes in wood and sediment storage were quantified for 13 streams that ranged from 4 to 144 years since the previous debris flow. The volume of wood and sediment in the channel, and the length of channel with exposed bedrock, were strongly correlated with the time since the previous debris flow. Wood increased the storage capacity of the channel and trapped the majority of the sediment in these steep headwater streams. In the absence of wood, channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for an extended period of time. With an adequate supply of wood, low‐order channels have the potential of storing large volumes of sediment in the interval between debris flows and can function as one of the dominant storage reservoirs for sediment in mountainous terrain. Copyright © 2003 John Wiley & Sons, Ltd.

Influence of step composition on step geometry and flow resistance in step‐pool streams of the Washington Cascades

Step‐pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step‐pool reaches without large woody debris (LWD) to 20 step‐pool reaches with LWD. Non‐LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water‐surface drop created by steps, larger grain sizes, and smaller Darcy‐Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non‐LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD‐loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step‐pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.

The characteristics of woody debris and sediment distribution in headwater streams, southeastern Alaska

Large woody debris (LWD), fine woody debris (FWD), fine organic debris (FOD), and sediment deposition were measured in 15 steep headwater streams with five management and disturbance regimes. Clear-cut channels logged in 1995 contained large accumulations of logging residue that initially provided sites for sediment storage. Half of the LWD in clear-cut channels was recruited during and immediately after logging. Woody debris from logging activities remains in young growth conifer channels 37 years after logging. Numbers of LWD in clear-cut and young conifer channels were significantly higher than in old-growth channels, although numbers of FWD pieces were not significantly different because of higher recruitment from old-growth stands. Channels that experienced recent (1979 and (or) 1993) and earlier (1961 and (or) 1979) scour and runout of landslides and debris flows contained less LWD and FWD, although large volumes of LWD and FWD were found in deposition zones. The volumes of sediment stored in young alder and recent landslide channels were higher than in the other channels. Because of the recruitment of LWD and FWD from young alder stands, the ratio of sediment stored behind woody debris to total sediment volume was higher in young alder channels compared with recent landslide channels. Numbers of LWD and FWD pieces in all streams were significantly correlated with the volumes of sediment stored behind woody debris. Timber harvesting and soil mass movement influence the recruitment, distribution, and accumulation of woody debris in headwater streams; this modifies sediment storage and transport in headwater channels.

Forest harvest patterns and landscape disturbance processes

A physically-based model of the topographic influence on debris flow initiation and a rule-based model for wind damage were used to assess the influence of forest clearcutting patterns (i.e., location, size, shape and distribution of cut units) on the potential for landscape disturbance by these processes in Charley Creek watershed, Washington State, USA. Simulated clearcutting patterns consisted of 7, 9 or 26 ha square or rectangular harvest units distributed in either an aggregated or dispersed pattern under three stream-buffering scenarios. The slope-stability model predicted that potentially unstable ground is concentrated along steep headwater streams and inner-gorge side-slopes. Areas susceptible to wind damage were determined from the combination of slope, aspect, elevation, soil drainage and primary tree species. Among the variables examined here, the location of harvest units constitutes the most important factor influencing the potential for shallow landsliding. In contrast, the location, size, and shape of clear cuts and the interactions among these three factors significantly influenced the potential for wind damage. Minimal correspondence between areas predicted to be potentially unstable and areas susceptible to wind damage implies that harvest patterns designed to mitigate the potential for shallow landsliding may not necessarily reduce the potential for wind damage. Our results demonstrate that: (1) the location of timber harvesting is more important than the geometry of harvest activity in influencing shallow landsliding; (2) forest harvest patterns strongly influence the potential for disturbance processes; and (3) a single cutting pattern will often fail to meet all landscape management goals.

Step‐Pool Streams: Adjustment to Maximum Flow Resistance

Steep headwater streams are often characterized by alternating steps and pools, which may be described by mean step heightand mean step length. A conceptual model is developed based on the notion that the largest floods are just capable of moving the largest debris in the channel. The model suggests that step pools evolve toward a condition of maximum flow resistance because maximum resistance implies maximum stability and that this condition is achieved when steps are regularly spaced and the mean step steepnessis slightly greater than the channel slopeS. To test this conceptual model, four series of flume experiments were performed. These experiments show that the relation between resistance to flow andis convex upward with maximum flow resistance occurring when steps are regularly spaced and havevalues between 1 and 2. Field measurements reveal that 18 natural step‐pool streams also satisfy the inequality, strongly suggesting that the form of such streams is adjusted to maximize resistance to flow. The results of the flume experiments are inconsistent with the proposition that step pools form as antidunes, as Froude numbers for the flume step pools at which flow resistance was maximized fall well below those values usually associated with these bed forms.

Transient Storage in Appalachian and Cascade Mountain Streams as Related to Hydraulic Characteristics

Hydraulic characteristics were measured in artificial streams and in 1st- to 5th-order streams in the Appalachian and Cascade mountains. Appalachian Mountain stream sites at Coweeta Hydrologic Laboratory, North Carolina, were on six 1st-order streams and a 1st- through 4th-order gradient of Ball Creek-Coweeta Creek. Cascade Mountain sites were located on constrained and unconstrained reaches of Lookout Creek, a 5th-order stream in H. J. Andrews Experimental Forest, Oregon. At each site, a tracer solution (chloride or rhodamine WT) was released for 30-180 min and then discontinued. At the downstream end of the release site, the resulting rise and fall of the tracer concentration was measured. These data, along with upstream concentration and measured widths and depths, were used in a computer model to estimate several hydraulic parameters including transient storage and lateral inflow. Estimated transient storage zone size (A_s) ranged from near zero in artificial streams to 2.0 m² in 5th-order streams. A_s was largest relative to surface cross-sectional area (A) at 1st-order sites where it averaged 1.2 × A, compared with 0.6 × A and 0.1 × A in unconstrained and constrained 5th-order sites, respectively. Where measured, lateral discharge inputs per metre of stream length ranged from 1.9% of instream discharge in 1st-order streams to 0.05% of instream discharge at 5th-order sites. Our results show that surface water exchange with storage zones is rapid and extensive in steep headwater streams and less extensive but still significant at 3rd- through 5th-order sites. An understanding of relationships between stream morphology, storage zone size, and extent of interactions between surface and subsurface waters will assist comparisons of solute dynamics in physically diverse streams.