Stream Shading Research Articles

Modelling the Contributions of Riparian Vegetation and Topography to Stream Shade Using LiDAR and Conventional Digital Elevation Data

ABSTRACTStream temperature is widely considered the master variable in stream ecosystems. One of the key drivers of diel and seasonal stream temperature variability is the solar radiation received at the stream surface, which can be influenced by shading associated with both larger scale topographic features and riparian vegetation. In this study, a stream shade model was developed that uses LiDAR point cloud data to model shading by riparian vegetation, including canopy overhang, and conventional elevation data to model stream shading by topography. The model was applied to a dominantly north–south oriented river flowing in a floodplain within a mountain valley. When compared with shade interpreted from PlanetScope visual imagery, the model predicted stream shade at the point scale with 92% agreement. Sources of error were attributed to pixel and azimuth band size, which can be refined within the model arguments, although at the cost of increased processing time. The shade model was re‐run after virtually rotating the reach by 90° and 270° clockwise to evaluate the effect of valley orientation. Peak reach‐wide sunlight exposure occurred approximately 2 h later in the day when the stream reach was rotated 90°, and produced greater shading from mid‐morning to mid‐afternoon. Further work should test the model on smaller streams using ground‐based oblique or drone‐based photography to provide ground‐truthing, particularly to assess the accuracy of predicted shade below over‐hanging vegetation.

How riparian and floodplain restoration modify the effects of increasing temperature on adult salmon spawner abundance in the Chehalis River, WA.

Stream temperatures in the Pacific Northwest are projected to increase with climate change, placing additional stress on cold-water salmonids. We modeled the potential impact of increased stream temperatures on four anadromous salmonid populations in the Chehalis River Basin (spring-run and fall-run Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and steelhead O. mykiss), as well as the potential for floodplain reconnection and stream shade restoration to offset the effects of future temperature increases. In the Chehalis River Basin, peak summer stream temperatures are predicted to increase by as much as 3°C by late-century, but restoration actions can locally decrease temperatures by as much as 6°C. On average, however, basin-wide average stream temperatures are expected to increase because most reaches have low temperature reduction potential for either restoration action relative to climate change. Results from the life cycle models indicated that, without restoration actions, increased summer temperatures are likely to produce significant declines in spawner abundance by late-century for coho (-29%), steelhead (-34%), and spring-run Chinook salmon (-95%), and smaller decreases for fall-run Chinook salmon (-17%). Restoration actions reduced these declines in all cases, although model results suggest that temperature restoration alone may not fully mitigate effects of future temperature increases. Notably, floodplain reconnection provided a greater benefit than riparian restoration for steelhead and both Chinook salmon populations, but riparian restoration provided a greater benefit for coho. This pattern emerged because coho salmon tend to spawn and rear in smaller streams where shade restoration has a larger effect on stream temperature, whereas Chinook and steelhead tend to occupy larger rivers where temperatures are more influenced by floodplain connectivity. Spring-run Chinook salmon are the only population for which peak temperatures affect adult prespawn survival in addition to rearing survival, making them the most sensitive species to increasing stream temperatures.

Passive Restoration of a Small Mountain Stream in Eastern Oregon

In this study, I documented the results of 11 years of passive restoration of a small mountain stream in northeastern Oregon. Removal of the main stressors on the system, grazing and agricultural practices, resulted in a strong recovery of the riparian vegetation over the middle two-thirds of the reach. Cover of trees and shrubs tripled, tree density increased sevenfold, and stream shading fourfold over the period 2007 through 2018. Average maximum daily stream temperature declined by 2.1 to 2.8 °C, and average maximum diurnal temperature range narrowed by 3.7 to 4.6 °C. Recolonizing American beavers (Castor canadensis) played an essential role in improving the condition of the streambed over 18% of the reach. Their dams and ponds initiated the process of streambed aggradation and transformed the single-thread, incised channel into a multi-thread configuration within beaver complexes. Vegetation expansion was much stronger in impounded than in un-impounded parts of the stream. Passive restoration was not effective in two sections, together comprising one-third of the reach. In the upper part of the stream, recovery stalled because of continued (unauthorized) trespass grazing during late summer and fall. The channelized lower section of the stream was too severely modified for measurable recovery to occur. I concluded that 11 years of passive restoration improved the functionality of the stream from 24% to 32%, and within beaver complexes, up to 57% of its pre-disturbance condition.

The effect of stream shading on the inflow characteristics in a downstream reservoir

AbstractIn thermally stratified reservoirs, inflows form density currents according to the interplay between inflow temperature and reservoir stratification. The temperature of inflowing water is affected by catchment properties, including shading by riparian vegetation. We hypothesize that the degree of shading in the catchment can affect the inflow dynamics in downstream reservoirs by changing inflow temperature and consequently the nature of the density current. We test it for a subtropical drinking water reservoir by combining catchment‐scale hydrological and stream temperature modeling with observations of reservoir stratification. We analyze the formation of density currents, defined as under, inter and overflow, for scenarios with contrasting shading conditions in the catchment. Inflow temperatures were simulated with the distributed water‐balance model LARSIM‐WT, which integrates heat‐balance and water temperature. River temperature measurements and simulations are in good agreement with a RMSE of 0.58°C. In simulations using the present state of shading, underflows are the most frequent flow path, 63% of the annual period. During the remaining time, river intrusion form interflows. In a scenario without stream shading, average inflow temperature increased by 2.2°C. Thus, interflows were the most frequent flow path (51%), followed by underflows (34%) and overflows (15%). With this change, we would expect a degradation of reservoir water quality, as overflows promote longer periods of anoxia and nutrient loads would be delivered to the photic zone, a potential trigger for algae blooms. This study revealed a potentially important, yet unexplored aspect of catchment management for controlling reservoir water quality.

Forest riparian buffers reduce timber harvesting effects on stream temperature, but additional climate adaptation strategies are likely needed under future conditions.

Stream water temperature imposes metabolic constraints on the health of cold-water fish like salmonids. Timber harvesting can reduce stream shading leading to higher water temperatures, while also altering stream hydrology. In the Pacific Northwest, riparian buffer requirements are designed to mitigate these impacts; however, anticipated future changes in air temperature and precipitation could reduce the efficacy of these practices in protecting aquatic ecosystems. Using a combined modeling approach (Soil and Water Assessment Tool (SWAT), Shade, and QUAL2K), this study examines the effectiveness of riparian buffers in reducing impacts of timber harvest on stream water temperature in Lookout Creek, Oregon across a range of potential future climates. Simulations assess changes in riparian management alone, climate alone, and combined effects. Results suggest that maximum stream water temperatures during thermal stress events are projected to increase by 3.3-7.4 °C due to hydroclimatic change alone by the end of this century. Riparian management is effective in reducing stream temperature increases from timber harvesting alone but cannot fully counteract the additional effects of a warming climate. Overall, our findings suggest that the protection of sensitive aquatic species will likely require additional adaptation strategies, such as the protection or provisioning of cool water refugia, to enhance survival during maximum thermal stress events.

Combined effects of an anthropogenic (forest harvesting) and natural (extreme rainfall event) disturbance on headwater streams in New Zealand

Abstract Although extreme hydrological events are a natural component of river ecosystem disturbance regimes, their frequency is predicted to increase with climate change. Anthropogenic activities have the potential to exacerbate the impact of such disturbances but there are few studies on the combined effects of both anthropogenic and extreme hydrological disturbances on stream ecosystems. We investigated the recovery of stream ecosystems over a 5‐year period following the impact of an anthropogenic (forest clear‐cut harvesting) and an extreme rainfall disturbance (estimated one‐in‐100 year average return interval) that generated debris flows in three headwater streams in New Zealand. Initially, most of the riparian vegetation was eliminated and showed little recovery 1 year later. Subsequent riparian recovery was led by wind‐borne, light‐demanding, pioneering exotic weed species, lengthening and altering the long‐term successional and recovery trajectories to a pre‐disturbance composition of indigenous shrubs. Stream shade, water temperature, and habitat had largely recovered after 5 years. However, the contribution of large wood to channel morphology and in‐stream habitat was compromised due to diminished wood supplies in the stream channel and a hiatus in up‐slope wood inputs until the riparian vegetation re‐establishes and the next crop of trees matures. After an initial decline, most indigenous fish taxa thrived in the post‐disturbance conditions, with significant increases in densities and biomass. The more sensitive fish taxa were scarce or absent, particularly those taxa that prefer pools with overhead and in‐stream cover provided by riparian vegetation and wood. Recovery of these taxa was outside the time frame of this study. Riffle dwelling fish communities were more resilient than pool dwelling fish communities. Invertebrate densities showed a similar response to fish. Post‐event invertebrate community composition differed from that typically found in post‐harvest headwater streams, comprising comparatively lower proportions of Chironomidae, Oligochaetes, and Mollusca taxa, and higher proportions of Trichoptera taxa. Progression toward pre‐event community composition was evident 5 years after the event. The compounding effect of forest removal from harvesting, along with riparian vegetation and wood removal by debris flows, lengthened the recovery of riparian vegetation and wood supplies with cascading effects on in‐stream habitat and biological communities.

Assessing the Functional Response to Streamside Fencing of Pastoral Waikato Streams, New Zealand

In New Zealand, streamside fencing is a well-recognised restoration technique for pastoral waterways. However, the response of stream ecosystem function to fencing is not well quantified. We measured the response to fencing of eight variables describing ecosystem function and 11 variables describing physical habitat and water quality at 11 paired stream sites (fenced and unfenced) over a 30-year timespan. We hypothesised that (1) fencing would improve the state of stream ecosystem health as described by physical, water quality and functional indicators due to riparian re-establishment and (2) time since fencing would increase the degree of change from impacted to less-impacted as described by physical, water quality and functional indicators. We observed high site-to-site variability in both physical and functional metrics. Stream shade was the only measure that showed a significant difference between treatments with higher levels of shade at fenced than unfenced sites. Cotton tensile-strength loss was the only functional measurement that indicated a response to fencing and increased over time since treatment within fenced sites. Our results suggest that stream restoration by fencing follows a complex pathway, over a space-for-time continuum, illustrating the overarching catchment influence at a reach scale. Small-scale (less than 2% of the upstream catchment area) efforts to fence the riparian zones of streams appear to have little effect on ecosystem function. We suggest that repeated measures of structural and functional indicators of ecosystem health are needed to inform robust assessments of stream restoration.

The effect of forestry management activities on stream water quality within a headwater plantation Pinus radiata forest

Many long-term forestry water quality studies focus on the impact of the harvesting phase of forestry operations. As a result, the water quality impacts of other activities such as forest (re)planting, thinning and fertilising are less well understood. Here we report the results from 23 years of monthly water quality monitoring from a steep headwater catchment within the Waikato region, New Zealand. Three experimental sub-catchments were planted in different proportions of Pinus radiata (PW2 = 100%, K-Pine 57%; PW3 = 36%). PW2 and PW3 were historical pastoral sites in the first rotation of plantation forestry and K-Pine was harvested and subsequently planted in a second rotation of forestry. All sites underwent two phases of stand thinning six and eight years after planting. The most significant effect of forestry was the increase in nitrate-N (NO3-N) and total nitrogen (TN) concentrations in response to both stand planting and stand thinning operations. Planting resulted in significant increases in NO3-N, TN and DRP over a four-year planting period at PW2. Significant increases in NO3-N and TN were detected over a four-year thinning period, at all three experimental sites. We propose that stream NO3-N and TN concentrations increased in response to stand planting at PW2 because of the change in physical conditions (e.g. increased stream shading, proliferation of nitrogen fixing weeds and decreased spatial extent of seepage wetlands) caused by the change in land use. Increasing nitrogen concentrations due to decreasing stream runoff could also be contributing a factor. Any decrease in NO3-N and TN concentrations in response to increased uptake by the rapidly growing P. radiata appears to have been masked by these other factors. However, the increase in NO3-N and TN concentrations in response to stand thinning at all three experimental sites suggests that NO3-N uptake by growing trees is an important process. Over the entire forestry period, median NO3-N concentrations at PW2 (first rotation site) increased by a factor four (400–1590 µg l−1). The median NO3-N during the post-thinning period was also one order of magnitude higher than those measured at K-Pine (second rotation site) during the post-thinning period. We attribute these differences to the NO3-N legacy within soil and/or groundwater of the previous sheep and beef cattle grazing land use. Long-term use of the PW2 catchment for forestry may eventually reduce NO3-N concentrations to the lower levels measured at K-Pine. Therefore, while NO3-N concentrations and loads may increase in response to the establishment of plantation forestry we envisage that these will decline if plantation forestry continues beyond the first rotation. This research highlights: (i) the complex nature of the response of stream water quality to catchment land use changes, and (ii) the value of studies that monitor the impact of land use/cover change over the long-term.

Modelling stream shade: 2. Predicting the effects of canopy shape and changes over time

Restoring riparian shade to formerly forested streams in cleared land is an important step in the restoration of stream ecological health. In a companion paper, we validated a numerical model for diffuse non-interceptance (difn) against measurements made in precisely-constructed physical models for two simplified geometries ‘canyon’ and ‘cylinder’ with trees lacking canopy ‘shape’. In this paper, we extend the model to include canopy ‘shape’ by representing trees as ellipses or kites, and quantifying shading from canopy overhang of the channel. The ratio of tree height (h) to stream width (w) strongly influences stream shade and is a key parameter when scoping riparian revegetation. Canopy width, overhang and viewpoint affect stream shade and could profitably be included in water temperature and aquatic plant growth models. Tree shape has a second-order effect on shade, most noticeably near the stream banks. The model predicts shade assuming a sky of uniform radiance (viz., it predicts shade to diffuse radiation) on the premise that difn is a useful index of long-term averaged light exposure. The model successfully reproduced the trajectory of shade reductions following riparian restoration predicted using a published physically-based model, confirming its suitability for scoping riparian shade restoration. While it would be feasible to include direct radiation in the model, this would require additional input data and computational effort.

Modelling stream shade: 1. Verifying numerical simulations with measurements on simple physical models

Sunlight exposure of streams controls their thermal regime and light availability for photosynthesis. Restoring riparian shade to formerly forested streams in cleared land is an important step in the restoration of stream ecological health. Methods exist for measuring existing stream shade, but management tools are needed for predicting future shade as riparian plantings grow. We refine an existing geometrically-based numerical model for diffuse lighting which can be linked to models of tree growth to investigate how shade changes over time. The model predicts ‘diffuse non-interceptance’ (difn) which is an index of long-term averaged lighting. Shade of real (meandering) streams is bounded by two computationally straightforward cases: a perfectly straight channel (canyon model) and a channel meandering so tightly that its shade geometry ‘collapses’ to that of a circular pool (cylinder model). In this paper, we verify model results against measurements of shading made with a canopy analyzer instrument in precisely-constructed physical models of both the cylinder and canyon cases. For both canyon and cylinder geometries the observed and predicted difn matched closely (over a wide range of the ratio tree height to stream width, h/w) corroborating the numerical model. As expected, light exposure at the channel mid-point was higher than at channel edges, highlighting the need to consider variations in lighting across the channel when predicting water temperature and aquatic plant growth. Over the range of h/w where difn changes rapidly, shade was appreciably higher for the cylinder than the canyon case.

Climate and land cover effects on the temperature of Puget Sound streams

AbstractWe apply an integrated hydrology‐stream temperature modeling system, DHSVM‐RBM, to examine the response of the temperature of the major streams draining to Puget Sound to land cover and climate change. We first show that the model construct is able to reconstruct observed historic streamflow and stream temperature variations at a range of time scales. We then explore the relative effect of projected future climate and land cover change, including riparian vegetation, on streamflow and stream temperature. Streamflow in summer is likely to decrease as the climate warms especially in snowmelt‐dominated and transient river basins despite increased streamflow in their lower reaches associated with urbanization. Changes in streamflow also result from changes in land cover, and changes in stream shading result from changes in riparian vegetation, both of which influence stream temperature. However, we find that the effect of riparian vegetation changes on stream temperature is much greater than land cover change over the entire basin especially during summer low flow periods. Furthermore, while future projected precipitation change will have relatively modest effects on stream temperature, projected future air temperature increases will result in substantial increases in stream temperature especially in summer. These summer stream temperature increases will be associated both with increasing air temperature, and projected decreases in low flows. We find that restoration of riparian vegetation could mitigate much of the projected summer stream temperature increases. We also explore the contribution of riverine thermal loadings to the heat balance of Puget Sound, and find that the riverine contribution is greatest in winter, when streams account for up to 1/8 of total thermal inputs (averaged from December through February), with larger effects in some sub‐basins. We project that the riverine impact on thermal inputs to Puget Sound will become greater with both urbanization and climate change in winter but become smaller in summer due to climate change. Copyright © 2016 John Wiley & Sons, Ltd.

A tool for assisting municipalities in developing riparian shade inventories

Streams in many urbanized watersheds have elevated water temperature, and stream temperature is a regulated aspect of water quality in many parts of the western U.S. Some municipal point source polluters develop temperature mitigation plans that involve compensatory measures linked to stream temperature, principally increasing riparian shade. In this paper we describe a tool designed to help municipalities and other riparian property managers develop inventories of current and potential stream shade. The tool is based on the insolation module of the published model Heat Source to which we made the following modifications: (1) developed user interfaces and regional supporting geospatial data that minimize data acquisition and user expertise requirements; (2) incorporated LiDAR data to produce direct high spatial resolution estimates of riparian vegetation architecture; (3) created a new method to estimate site specific potential vegetation, with concomitant changes in the shading algorithm involving finer discretization and fewer parameter requirements. The new tool produces estimates of existing shade conditions comparable to those derived from on the ground field measurements for small to medium sized streams. We used the tool to develop a riparian shade inventory and a map of potential shade restoration for the City of Albany, Oregon, U.S.A. Results of this case study demonstrate that the tool can help municipalities meet shade reporting and monitoring goals at a minimum cost, including identifying specific areas with highest potential gains in shade. However, the case study identified limitations in some regional GIS framework data that potentially limit broad applicability or increase costs.

Adapting streams for climate change using riparian broadleaf trees and its consequences for stream salmonids

Summary The societal value, ecological importance and thermal sensitivity of stream‐dwelling salmonids have prompted interest in adaptive management strategies to limit the effects of climate change on their habitats. Additionally, in northern temperate regions, the management and restoration of riparian broadleaf forest is advocated increasingly to dampen variations in stream water temperature and discharge, but might have collateral effects on salmonids by changing allochthonous subsidies. Here, in a cross‐sectional analysis of 18 temperate headwaters with different riparian and catchment land use, we use classical fisheries data alongside stable isotope ratios in salmonids and their macroinvertebrate prey to examine whether increasing catchment cover of broadleaf trees could (i) increase the density, biomass and size of salmonids, (ii) increase brown trout (Salmo trutta) dietary reliance on production of terrestrial origin and (iii) mediate allochthonous energy flux between aquatic macroinvertebrates and brown trout. Contrary to expectation, catchment broadleaf cover had no systematic effect on salmonid density or individual size, although salmonid biomass was lowest in streams draining non‐native conifers. Moreover, there was no major effect of land use on the dependence of S. trutta on terrestrial production: averaged across all sites, trout used more production from in‐stream (62 ± 3%: mean ± 1 SE) than terrestrial (38 ± 3%) sources. Dependence on terrestrial production varied more substantially among individual streams than with riparian land use, mirroring site‐specific patterns observed in macroinvertebrates. Although increased broadleaf cover could benefit salmonids by offsetting the impacts of warming related to climate change, these results imply that broadleaf restoration along temperate, upland headwaters is neutral with respect to salmonid biomass, density and terrestrial subsidies. In contrast, the use of non‐native conifers for stream shading could have negative effects on salmonid production. Knowledge of the ecological implications of climate change adaptation remains rudimentary, and we advocate further evaluations like ours not only for fresh waters, but for ecosystems more generally.

Fatty Acid Composition at the Base of Aquatic Food Webs Is Influenced by Habitat Type and Watershed Land Use

Spatial variation in food resources strongly influences many aspects of aquatic consumer ecology. Although large-scale controls over spatial variation in many aspects of food resources are well known, others have received little study. Here we investigated variation in the fatty acid (FA) composition of seston and primary consumers within (i.e., among habitats) and among tributary systems of Lake Michigan, USA. FA composition of food is important because all metazoans require certain FAs for proper growth and development that cannot be produced de novo, including many polyunsaturated fatty acids (PUFAs). Here we sampled three habitat types (river, rivermouth and nearshore zone) in 11 tributaries of Lake Michigan to assess the amount of FA in seston and primary consumers of seston. We hypothesize that among-system and among-habitat variation in FAs at the base of food webs would be related to algal production, which in turn is influenced by three land cover characteristics: 1) combined agriculture and urban lands (an indication of anthropogenic nutrient inputs that fuel algal production), 2) the proportion of surface waters (an indication of water residence times that allow algal producers to accumulate) and 3) the extent of riparian forested buffers (an indication of stream shading that reduces algal production). Of these three land cover characteristics, only intense land use appeared to strongly related to seston and consumer FA and this effect was only strong in rivermouth and nearshore lake sites. River seston and consumer FA composition was highly variable, but that variation does not appear to be driven by the watershed land cover characteristics investigated here. Whether the spatial variation in FA content at the base of these food webs significantly influences the production of economically important species higher in the food web should be a focus of future research.

Simulating the Effects of Forest Management on Stream Shade in Central Idaho

Simulating the Effects of Forest Management on Stream Shade in Central Idaho

Variable-retention riparian harvesting effects on riparian air and water temperature of sub-boreal headwater streams in British Columbia

A 5-year (2002–2006) before–after control impact study was initiated in three watersheds of the British Columbia central interior to assess the ability of a variable retention riparian treatment to maintain fish habitat conditions in small sub-boreal streams (<2m width). This paper presents findings for the stream shade and air and stream temperature component of the study. Eight streams were studied to assess stream shade, riparian air, and stream water temperature response during summer months to a policy retention level of at least 10stems of merchantable timber per 100m of channel length. After harvesting there was a significant decrease in shade as well as an increase in air and stream temperature at all treatment sites. Riparian harvesting reduced stream shade by 30–50% from pre-harvest levels but shade was recovering to pre-harvest levels 2–3years after harvesting. Mean weekly average and maximum air temperatures at treatment sites increased more than 3°C compared to control locations. Mean weekly average and maximum stream temperatures at treatment sites increased by as much as 5 and 6°C, respectively. Despite the recovery of shade measured at the water surface, mean and maximum water temperatures remained significantly higher at treatment sites than control sites. The discrepancy between shade recovery and temperature response indicates that vegetative surface height receiving radiation must be considered along with shade. Shade from overstory may be more effective at maintaining riparian air and stream temperatures than lower understory because it can limit energy transfer to lower layers of the forest canopy and ground surface.

Modeled riparian stream shading: Agreement with field measurements and sensitivity to riparian conditions

Summary Shading by riparian vegetation and streambanks reduces incident solar radiation on channels, and accurate estimation of riparian shading through the sun’s daily arc is a critical aspect of water temperature and dissolved oxygen modeling. However, riparian trees exhibit complex shapes, often leaning and growing branches preferentially over channels to utilize the light resource. As a result, riparian vegetation cast complex shadows with significant variability at the scale of meters. Water quality models necessarily simplify factors affecting shading at the expense of accuracy. All models must make simplifying assumptions about tree geometry. Reach-based models must average channel azimuth and riparian conditions over each reach, and GIS models must also accept errors in the channel-riparian relationships caused by the DEM grid detail. We detail minor improvements to existing shade models and create a model (SHADE2) that calculates shading ratio (%) by riparian canopy at any time and location for given stream characteristics including stream azimuth, stream width, canopy height, canopy overhang, and height of maximum canopy overhang. Sensitivity of simulated shade to these variables is explored. We also present a new field photographic technique for quantifying shade and use this technique to provide data to test the SHADE2 algorithm. Twenty-four independent shade measurements were made in eight channels with mature hardwood riparian trees at different times of the summer and at different times of the day. Agreement between measured and modeled shade was excellent, with r 2 of 0.90.

Accuracy of Removal Electrofishing Estimates of Trout Abundance in Rocky Mountain Streams

AbstractRemoval electrofishing is frequently used to estimate fish distribution and abundance in streams because it is simple and requires only one visit to a site. However, because the removal method usually overestimates capture efficiency and therefore underestimates fish abundance, some biologists have questioned its use in favor of less biased methods. In southern Idaho streams in the summers of 2006 and 2007, rainbow trout Oncorhynchus mykiss, cutthroat trout O. clarkii, and brook trout Salvelinus fontinalis were captured with backpack electrofishers using pulsed DC and marked and released in blocknetted sites; on the following day, four‐pass electrofishing removals were conducted. Removal electrofishing underestimated the abundance of trout 10 cm and larger by 17% (four passes), 22% (three), and 25% (two); for trout less than 10 cm, the respective underestimates were 27, 27, and 37%. Removal estimates were biased in part because capture efficiency progressively decreased for fish 10 cm and larger from 58% in pass one to 37% (two passes), 30% (three), and 18% (four); a similar decline was not as evident for fish less than 10 cm. Increased channel complexity, in the form of boulder substrate, water depth, and stream shading, increased bias in removal estimates. Linear regression models incorporating these and other variables explained 44–67% of the variation in this bias. Visiting new sites in the summer of 2009 with a new field crew produced similar amounts of removal estimate bias, but predictions based on multiple‐regression results did not correct the bias any more accurately than did using a mean correction rate from the original sites. Our results suggest that multiple‐pass removal sampling in typical Rocky Mountain streams can produce population estimates that are consistently but not drastically biased and are, therefore, probably adequate for most basic fish population monitoring even without correction, especially if electrofisher settings and crew training balance the need to minimize injury with effective fish sampling.Received January 4, 2011; accepted June 20, 2011

Response of vegetation, shade and stream temperature to debris torrents in two western Oregon watersheds

This study examines watershed patterns of riparian vegetation, shade, and stream temperature eight years after extreme storm events triggered numerous debris torrents throughout the Pacific Northwest. We examined twelve impacted streams in two western Oregon watersheds: the Calapooia River in the western Cascades and the Williams River in the Coast Range. Red alder ( Alnus rubra) and willow ( Salix spp.) were the dominant species on debris torrented areas in both watersheds. Post-disturbance vegetation recovery was significant in both watersheds, impacting shade and stream temperatures. However, red alder density, basal area, and height were significantly greater along streams in the Williams River watershed than along streams in the Calapooia River watershed. Willow density, basal area and height were similar between the watersheds. Stream shading levels mirrored red alder growth, with greater average shading in the Williams River watershed. The greater shade translated into lower summer maximum stream temperatures and maximum diurnal stream temperature fluctuations in the Williams River as compared to the Calapooia River watershed. Minimum stream temperatures were not different between the two watersheds. The rapid re-growth of red alder along the Williams River watershed ultimately lead to a rapid decline in maximum summer stream temperatures for that watershed compared to the Calapooia River watershed. The location where the disturbance occurred had an important role in determining the rate and pathway of stream recovery.

Environmental factors influencing the species diversity of macrophytes in middle-sized streams in Latvia

The species diversity of macrophytes and their abundance in middle-sized streams of Latvia were investigated. On the basis of environmental factors (stream width, water depth, substrate type, shading, and flow velocity), five major groups of streams were distinguished representing mutually exclusive macrophyte communities—(1) fast flowing streams on gravel substrates, (2) slow flowing streams on gravel substrates, (3) fast flowing streams on sandy substrates, (4) slow flowing streams on sandy substrates, and (5) streams with soft, silty substrates. A total of 48 macrophyte taxa were found in 72 surveyed sites. The most frequent species in the investigated streams were Sparganium emersum, S. erectum s.l., Nuphar lutea, Veronica beccabunga, as well as the invasive alien Elodea canadensis. The species richness ranged from 2 to 22 per site, and Shannon diversity index varied from 0.61 to 2.88. The highest species richness (22) was found in slow flowing streams with gravel substrates. Poor macrophyte composition was characteristic for fast flowing streams on sandy substrates.