Wood in neotropical headwater streams, Costa Rica

Department Head: Sally J. Sutton.2010 Summer.Includes bibliographical references.Wood has been shown to be an integral component of forest streams throughout the temperate climate zone, both in terms of the physical structure of the channel and in terms of aquatic ecosystem function, but the function of wood in undisturbed tropical streams has not been studied. This dissertation represents the first systematic analysis of instream wood in a tropical setting to be published. This study was limited to the headwater streams (drainage area <8.5 km2) of La Selva Biological Station, on the Atlantic margin of Costa Rica, a wet tropical site with limited landslide activity. Although the results are instructive and enable comparisons with the vast temperate instream wood literature, they should not be construed as representative of debris flow-dominated wet tropical forest streams or of dry or seasonal tropical forest streams. Wood loads in the thirty 50-m-long study reaches examined ranged from 3.0 to 34.7 m3 of wood per 100 m of channel length and 41 to 612 m3 of wood per ha of channel area. Average values are 12.3 m3/100 m and 189 m3/ha. These values fall generally in the lower range of wood load reported for temperate streams, with values typically lower than those reported from the Pacific Northwest region and the Great Lakes region and within the range of those reported from the Rocky Mountain region and from Southern Hemisphere study sites. Comparisons to study sites in eastern North America, Europe, and Japan are problematic because La Selva is a generally undisturbed forest, whereas studies from those regions are conducted in streams with significant human impact and tend to have very small wood loads. Flow hydraulics appear to be the dominant control on the lateral distribution of wood in the channels of La Selva, but they are only a partial control on the longitudinal distribution of wood, explaining about half of the variation in wood load among the study sites. The remainder of the variation is likely caused by the stochastic nature of large tree fall. In spite of the high temporal variability of lateral input of wood to the channels, spatial variability is small, partially because of the paucity of landslides at La Selva. Therefore, I propose that instream transport has a greater influence on the longitudinal distribution of wood than lateral input variability. Wood in a representative subset of 10 of the 50-m-long study reaches was monitored for 2.3 years. The wood in the streams of La Selva is more transient than wood in most sites studied in the temperate zone, with piecewise mean residence times ranging from 2 to 12 years and volume-wise mean residence times ranging from 2 to 83 years among the 10 sites monitored. Average values were 5 and 7 years, respectively. These are roughly an order of magnitude shorter than mean residence times reported from the Pacific Northwest, but similar to times reported from the Colorado Rocky Mountains. The short residence times may be a result of more frequent large floods caused by the wet tropical climate, higher decay rates caused by the warm tropical climate, or both. Perhaps because of this transience, wood was found to have minimal influence on flow resistance in a subset of 6 of the 50-m-long study reaches. In contrast, wood has been shown to be a major control on flow resistance in temperate mountain streams. It is possible that the channel geometry and bed material size are adjusted to the frequent high discharges, which also mobilize and rework the wood, causing grain and form resistance to overwhelm any resistance contribution from wood. Instream wood at La Selva also appears to have a minimal influence on sediment transport. Jams in sand-bed channels and jams in boulder-bed channels had no associated residual elevation drop. Jams in gravel-bed channels did alter bed elevation by trapping sediment wedges behind them, but analysis of tracer clast movement at one gravel-bed jam resulted in no observable difference in transport distances or mobility between clasts placed upstream of the jam and those placed downstream. An additional forest-stream interaction that was documented is diel cycles in stream discharge associated with groundwater withdrawal by the forest for evapotranspiration. Analysis of the cycles indicates a strong correlation with vapor pressure differential, which previous researchers have found to correlate with sap flow. Further analysis of the cycles suggests that at low-stage conditions transmissivity dominates groundwater flow into the channel, while at high-stage conditions hydraulic gradient is dominant

Controls on the diurnal streamflow cycles in two subbasins of an alpine headwater catchment

In high-altitude alpine catchments, diurnal streamflow cycles are typically dominated by snowmelt or ice melt. Evapotranspiration-induced diurnal streamflow cycles are less observed in these catchments but might happen simultaneously. During a field campaign in the summer 2012 in an alpine catchment in the Swiss Alps (Val Ferret catchment, 20.4 km2, glaciarized area: 2%), we observed a transition in the early season from a snowmelt to an evapotranspiration-induced diurnal streamflow cycle in one of two monitored subbasins. The two different cycles were of comparable amplitudes and the transition happened within a time span of several days. In the second monitored subbasin, we observed an ice melt-dominated diurnal cycle during the entire season due to the presence of a small glacier. Comparisons between ice melt and evapotranspiration cycles showed that the two processes were happening at the same times of day but with a different sign and a different shape. The amplitude of the ice melt cycle decreased exponentially during the season and was larger than the amplitude of the evapotranspiration cycle which was relatively constant during the season. Our study suggests that an evapotranspiration-dominated diurnal streamflow cycle could damp the ice melt-dominated diurnal streamflow cycle. The two types of diurnal streamflow cycles were separated using a method based on the identification of the active riparian area and measurement of evapotranspiration

A flume experiment on wood storage and remobilization in braided river systems

This work investigates wood dynamics in braided streams through physical modelling in a mobile bed laboratory flume, with the specific objective to characterize wood storage and turnover as a function of wood input rate and of wood element type. Three parallel channels (1.7 m wide, 10 m long) filled with uniform sand were used to reproduce braided networks with constant water discharge and sediment feeding. Wood dowels with and without simplified root wads were regularly added at the upstream end of each flume at different input rates, with a 1:2:3 ratio between the three flumes. Temporal evolution of wood deposition patterns and remobilization rates were monitored by a series of vertical images that permitted the recognition of individual logs. Results show that wood tends to disperse in generally small accumulations ( 10 elements) that are less prone to remobilization. Presence of root wads seems to play a minor role in wood deposition, but it reduces the average travel distance of logs. Turnover rates of logs were similar in the three flumes, independently of wood input rate and largely resembling the turnover rate of exposed bars. For the simulated conditions, significant effects of wood on bed morphology were not observed, suggesting that interactions with fine sediments and living vegetation are crucial to form large, stable wood jams able to bring about relevant morphological change

INTEGRAL/IBIS 7-year All-Sky Hard X-Ray Survey. Part II: Catalog of Sources

This paper is the second in a series devoted to the hard X-ray (17-60 keV) whole sky survey performed by the INTEGRAL observatory over seven years. Here we present a catalog of detected sources which includes 521 objects, 449 of which exceed a 5 sigma detection threshold on the time-averaged map of the sky, and 53 were detected in various subsamples of exposures. Among the identified sources with known and suspected nature, 262 are Galactic (101 low-mass X-ray binaries, 95 high-mass X-ray binaries, 36 cataclysmic variables, and 30 of other types) and 219 are extragalactic, including 214 active galactic nuclei (AGNs), 4 galaxy clusters, and galaxy ESO 389-G 002. The extragalactic (|b|>5 deg) and Galactic (|b|<5 deg) persistently detected source samples are of high identification completeness (respectively ~96% and ~94%) and valuable for population studies.Comment: 17 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

The influence of large woody debris on post-wildfire debris flow sediment storage

Debris flows transport large quantities of water and granular material, such as sediment and wood, and this mixture can have devastating effects on life and infrastructure. The proportion of large woody debris (LWD) incorporated into debris flows can be enhanced in forested areas recently burned by wildfire because wood recruitment into channels accelerates in burned forests. In this study, using four small watersheds in the Gila National Forest, New Mexico, which burned in the 2020 Tadpole Fire, we explored new approaches to estimate debris flow velocity based on LWD characteristics and the role of LWD in debris flow volume retention. To understand debris flow volume model predictions, we examined two models for debris flow volume estimation: (1) the current volume prediction model used in US Geological Survey debris flow hazard assessments and (2) a regional model developed to predict the sediment yield associated with debris-laden flows. We found that the regional model better matched the magnitude of the observed sediment at the terminal fan, indicating the utility of regionally calibrated parameters for debris flow volume prediction. However, large wood created sediment storage upstream from the terminal fan, and this volume was of the same magnitude as the total debris flow volume stored at the terminal fans. Using field and lidar data we found that sediment retention by LWD is largely controlled by channel reach slope and a ratio of LWD length to channel width between 0.25 and 1. Finally, we demonstrated a method for estimating debris flow velocity based on estimates of the critical velocity required to break wood, which can be used in future field studies to estimate minimum debris flow velocity values.</p

Plants as river system engineers

I would like to acknowledge three research grants/contracts that are supporting my current research on this theme: Grant F/07 040/AP from the Leverhulme Trust; Grant NE/F014597/1 from the Natural Environment Research Council, UK, and the REFORM collaborative project funded by the European Union Seventh Framework Programme under grant agreement 282656

Modeling the interactions between river morphodynamics and riparian vegetation

The study of river-riparian vegetation interactions is an important and intriguing research field in geophysics. Vegetation is an active element of the ecological dynamics of a floodplain which interacts with the fluvial processes and affects the flow field, sediment transport, and the morphology of the river. In turn, the river provides water, sediments, nutrients, and seeds to the nearby riparian vegetation, depending on the hydrological, hydraulic, and geomorphological characteristic of the stream. In the past, the study of this complex theme was approached in two different ways. On the one hand, the subject was faced from a mainly qualitative point of view by ecologists and biogeographers. Riparian vegetation dynamics and its spatial patterns have been described and demonstrated in detail, and the key role of several fluvial processes has been shown, but no mathematical models have been proposed. On the other hand, the quantitative approach to fluvial processes, which is typical of engineers, has led to the development of several morphodynamic models. However, the biological aspect has usually been neglected, and vegetation has only been considered as a static element. In recent years, different scientific communities (ranging from ecologists to biogeographers and from geomorphologists to hydrologists and fluvial engineers) have begun to collaborate and have proposed both semiquantitative and quantitative models of river-vegetation interconnections. These models demonstrate the importance of linking fluvial morphodynamics and riparian vegetation dynamics to understand the key processes that regulate a riparian environment in order to foresee the impact of anthropogenic actions and to carefully manage and rehabilitate riparian areas. In the first part of this work, we review the main interactions between rivers and riparian vegetation, and their possible modeling. In the second part, we discuss the semiquantitative and quantitative models which have been proposed to date, considering both multi- and single-thread river

The Effects of Replacing Native Forest on the Quantity and Impacts of In-Channel Pieces of Large Wood in Chilean Streams

Dead trees in rivers can significantly affect their morphological and ecological properties by increasing flow resistance, affecting sediment transport, and storing organic matter. Logs are usually recruited from banks or along the entire upstream basin. Although it is generally acknowledged that forested headwater streams feature higher volumes of in-channel pieces of large wood, the influence of forest type and forest management of the potential recruitment zone on the volumes and effects of wood have been less explored, especially in relation to the effects of replacing native forests with pine plantations. This paper presents a comparison of volumes of wood, and characteristics and effects on streams draining paired basins with comparable slopes, areas, and hydrologic regimes, but different in terms of land use. The five selected pairs of basins are located in the Coastal and Andean mountain Ranges in central Chile, in order to compare native forest and pine plantation basins. The results show that logs tend to be shorter and with larger diameters in streams draining native forest basins. Because of their smaller dimensions, logs and jams tend to be more mobile and oriented parallel to the flow. Volumes of in-channel wood in native forest basins are only slightly larger than in pine plantation basins, and no differences have been identified in terms of morphological effects on channel geometry. Also, fish type and biomass were comparable among pairs. Evidence highlights the importance of the width of riparian buffers in mitigating the effects of land use change, especially the substitution of native forest with plantations