Stream Surface Area Research Articles

The Geometry of Flow: Advancing Predictions of River Geometry With Multi‐Model Machine Learning

AbstractHydraulic geometry parameters describing river hydrogeomorphic relationships are critical for determining a channel's capacity to convey water and sediment which is important for flood forecasting. Although well‐established, power‐law hydraulic geometry curves have been widely used to understand riverine systems and mapping flooding inundation worldwide for the past 70 years, we have become increasingly aware of their limitations. In the present study, we have moved beyond these traditional power‐law relationships, testing the ability of machine‐learning models to provide improved predictions of river width and depth. For this work, we have used an unprecedentedly large river measurement data set (HYDRoSWOT) as well as a suite of watershed predictor data to develop novel data‐driven approaches to better estimate river geometries over the contiguous United States (CONUS). Our Random Forest, XGBoost, and neural network models out‐performed the traditional, regionalized power law‐based hydraulic geometry equations for both width and depth, providing R‐squared values of as high as 0.75 for width and as high as 0.67 for depth, compared with R‐squared values of 0.45 for width and 0.18 for depth from the regional hydraulic geometry equations. Our results also show diverse performance outcomes across stream orders and geographical regions for the different machine‐learning models, demonstrating the value of using multi‐model approaches to maximize the predictability of river geometry. The developed models have been used to create the newly publicly available STREAM‐geo data set, which provides river width, depth, width/depth ratio, and river and stream surface area (%RSSA) for nearly 2.7 million NHDPlus stream reaches across the contiguous US.

Mapping proglacial headwater streams in High Mountain Asia using PlanetScope imagery

Headwater streams transport nutrients, sediment, and mineral-rich groundwater downstream. In High Mountain Asia (HMA), headwater streams also funnel glacier and snow melt to sustain continuous water supply for the downstream region. These channels remain poorly mapped because of their inaccessibility and because they are smaller than the resolution of Landsat (30 m) and Sentinel-2 (10 m). In this study, we assessed the ability of 3 m resolution PlanetScope imagery to detect the proglacial headwaters downstream of all high-altitude glaciers larger than 5 km2 in HMA. We created 3000 manually labeled image tiles to train and evaluate computer vision (CV) against techniques common in the hydrologic remote sensing literature, specifically normalized difference water index (NDWI) thresholding and random forests (RF). Results indicate that CV best detects the headwater streams with >0.60 F1-scores, nearly 0.20 points higher than RF and 0.45 points higher than thresholding. We also assessed how errors in CV propagate to derived hydrologic information, exemplified by the biogeochemically critical measurement of stream surface area. We found that CV classifications produced surface areas with 0.98 R2, 0.01 km2 MAE, and 0.02 km2 RMSE against manually labeled surface areas. We also observed the best CV performance during the spring season with 30% more skillful classification performance than in summer and fall. Our results prove the ability of PlanetScope imagery to detect and map headwater streams accurately and at scale, and that classification errors stemming from the imagery or the CV methods do not greatly impair our ability to quantify stream surface area meaningful for biogeochemical exchange and hydrology studies.

Spatiotemporal Dynamics of CO2 Gas Exchange From Headwater Mountain Streams

AbstractMountain streams play an important role in the global carbon cycle by transporting, metabolizing, and exchanging carbon they receive from the terrestrial environment. The rates at which these processes occur remain highly uncertain because of a paucity of observations and the difficulty of measuring gas exchange rates in steep, turbulent mountain streams. This uncertainty is compounded by large temporal and spatial variability in stream carbon dioxide (CO2) concentrations in mountain environments. In this study, we measured diel, seasonal, and annual variations in CO2 partial pressure (pCO2) in seven headwater streams and a groundwater spring in the Colorado Rocky Mountains to determine how CO2 exchange fluxes () vary with time, annual precipitation, and landscape characteristics. Our results show that temporal variability in pCO2 and in mountain streams is large and is strongly influenced by solar radiation, the accumulation and melting of seasonal snowpacks, and interannual variations in precipitation. Spatial variations in pCO2 and were related to landscape characteristics, with soil organic matter, wetlands, and likely groundwater discharge zones having a positive influence. Periglacial features, such as ice and rock glaciers, had a negative influence on stream pCO2 and . Estimated from streams in an alpine/subalpine region of Colorado was 3.4 kg C m−2 yr−1 normalized to stream surface area (95% CI: 2.1–5.0 kg C m−2 yr−1), consistent with recent work on CO2 exchange from mountain streams in the Swiss Alps. Our results highlight the importance of mountain streams as substantial contributors in the global carbon cycle.

Temporally Variable Stream Width and Surface Area Distributions in a Headwater Catchment

AbstractHeadwater stream networks expand and contract in response to event‐driven and seasonal catchment wetness conditions. This dynamic behavior drives variability in the width, length, and surface area of streams, important parameters for constraining a range of ecological and biogeochemical processes, such as atmospheric gas exchange. While the longitudinal expansion and contraction of streams has been studied for some time, variability in stream widths remains poorly understood. Recent studies have found that stream widths at average baseflow conditions follow a log‐normal frequency distribution across diverse physiographies. To examine how the distribution of widths varies with flow conditions, we surveyed stream widths 12 times across a 48.4‐ha research watershed, located in the Duke Forest in central North Carolina, USA. Here, we show that as runoff increased from the 37th to 99th percentiles of flow, flowing streams widened across the network (“lateral expansion”) and streamflow simultaneously extended upstream to reactivate dry channels (“longitudinal expansion”). In general, as runoff increased, the marginal increase in stream surface area was equally divided between longitudinal and lateral expansion. Even so, the median stream width widens on average with increasing runoff, suggesting that longitudinal and lateral expansion affect the distribution of stream width differently. We find that the form of the relationship between stream width and runoff is a power law, which can be used to refine models for surface area estimation.

Temporally Variable Stream Width and Surface Area Distributions in a Headwater Catchment

Temporally Variable Stream Width and Surface Area Distributions in a Headwater Catchment

Meltwater temperature in streams draining Alpine glaciers

Climate change is significantly impacting lotic environments, through changes to hydrology, biodiversity and species distribution. Effects of climate change are greatest at high elevation and biota in and around glacier-fed rivers is likely, therefore, to be at great risk. How climate change influences hydrology will have great impact on river water temperature as glacier-fed rivers in Alpine environments are extremely sensitive to climatic change. This paper assesses five rivers: Four glacier-fed rivers (36.9–83.7% basin glacierisation) located in the Swiss Alps, and one located in an ice-free catchment in the Bernese Oberland, Switzerland. The aim was to assess the impact of basin characteristics on river water temperature. A distinct paradoxical relationship was identified whereby water temperature in some glacier-fed rivers was reduced during the time of highest incoming shortwave radiation receipts and high air temperature. Whether a summer cooling effect presented itself in all glacier-fed rivers within this study was researched. The key findings were that the identified summer cooling effect was not present in all rivers, despite percentage glacierisation. Percentage glacier cover has often been reported as they key determiner of water temperature in such rivers. More important was the stream dimensions, notably stream surface area. Understanding the controlling factors that influence water temperature of glacier-fed rivers will help river managers and planners in understanding how climate change will affect fisheries downstream of glaciers over the coming decades. This may allow plans to be introduced to try and mitigate warmer water temperature that will result, in some glacier-fed rivers, as the climate warms.

Applying High-Resolution Imagery to Evaluate Restoration-Induced Changes in Stream Condition, Missouri River Headwaters Basin, Montana

Degradation of streams and associated riparian habitat across the Missouri River Headwaters Basin has motivated several stream restoration projects across the watershed. Many of these projects install a series of beaver dam analogues (BDAs) to aggrade incised streams, elevate local water tables, and create natural surface water storage by reconnecting streams with their floodplains. Satellite imagery can provide a spatially continuous mechanism to monitor the effects of these in-stream structures on stream surface area. However, remote sensing-based approaches to map narrow (e.g., <5 m wide) linear features such as streams have been under-developed relative to efforts to map other types of aquatic systems, such as wetlands or lakes. We mapped pre- and post-restoration (one to three years post-restoration) stream surface area and riparian greenness at four stream restoration sites using Worldview-2 and 3 images as well as a QuickBird-2 image. We found that panchromatic brightness and eCognition-based outputs (0.5 m resolution) provided high-accuracy maps of stream surface area (overall accuracy ranged from 91% to 99%) for streams as narrow as 1.5 m wide. Using image pairs, we were able to document increases in stream surface area immediately upstream of BDAs as well as increases in stream surface area along the restoration reach at Robb Creek, Alkali Creek and Long Creek (South). Although Long Creek (North) did not show a net increase in stream surface area along the restoration reach, we did observe an increase in riparian greenness, suggesting increased water retention adjacent to the stream. As high-resolution imagery becomes more widely collected and available, improvements in our ability to provide spatially continuous monitoring of stream systems can effectively complement more traditional field-based and gage-based datasets to inform watershed management.

Groundwater data improve modelling of headwater stream CO<sub>2</sub> outgassing with a stable DIC isotope approach

Abstract. A large portion of terrestrially derived carbon outgasses as carbon dioxide (CO2) from streams and rivers to the atmosphere. Particularly, the amount of CO2 outgassing from small headwater streams is highly uncertain. Conservative estimates suggest that they contribute 36 % (i.e. 0.93 petagrams (Pg) C yr−1) of total CO2 outgassing from all fluvial ecosystems on the globe. In this study, stream pCO2, dissolved inorganic carbon (DIC), and δ13CDIC data were used to determine CO2 outgassing from an acidic headwater stream in the Uhlířská catchment (Czech Republic). This stream drains a catchment with silicate bedrock. The applied stable isotope model is based on the principle that the 13C ∕ 12C ratio of its sources and the intensity of CO2 outgassing control the isotope ratio of DIC in stream water. It avoids the use of the gas transfer velocity parameter (k), which is highly variable and mostly difficult to constrain. Model results indicate that CO2 outgassing contributed more than 80 % to the annual stream inorganic carbon loss in the Uhlířská catchment. This translated to a CO2 outgassing rate from the stream of 34.9 kg C m−2 yr−1 when normalised to the stream surface area. Large temporal variations with maximum values shortly before spring snowmelt and in summer emphasise the need for investigations at higher temporal resolution. We improved the model uncertainty by incorporating groundwater data to better constrain the isotope compositions of initial DIC. Due to the large global abundance of acidic, humic-rich headwaters, we underline the importance of this integral approach for global applications.

Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed

Streams and rivers emit significant amounts of CO2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO2 degassing based on the water-air CO2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (δ13C-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on DIC concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO2 and DIC and an increase in δ13C-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater 12C-DIC compared to 13C-DIC. This downstream enrichment in 13C-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of 13C in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO2 concentrations, we quantified CO2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2–4, our CO2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO2, gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.

Measurements of coarse particulate organic matter transport in steep mountain streams and estimates of decadal CPOM exports

Summary Coarse particulate organic matter (CPOM) provides a food source for benthic organisms, and the fluvial transport of CPOM is one of the forms in which carbon is exported from a forested basin. However, little is known about transport dynamics of CPOM, its relation to discharge, and its annual exports from mountain streams. Much of this knowledge gap is due to sampling difficulties. In this study, CPOM was sampled over one-month snowmelt high flow seasons in two high-elevation, subalpine, streams in the Rocky Mountains. Bedload traps developed for sampling gravel bedload were found to be suitable samplers for CPOM transport. CPOM transport rates were well related to flow in consecutive samples but showed pronounced hysteresis over the diurnal fluctuations of flow, between consecutive days, and over the rising and falling limbs of the high-flow season. In order to compute annual CPOM load, hysteresis effects require intensive sampling and establishing separate rating curves for all rising and falling limbs. Hysteresis patterns of CPOM transport relations identified in the well-sampled study streams may aid with estimates of CPOM transport and export in less well-sampled Rocky Mountain streams. Transport relations for CPOM were similar among three high elevation mountain stream with mainly coniferous watersheds. Differences among streams can be qualitatively attributed to differences in CPOM contributions from litter fall, from the presence of large woody debris, its grinding into CPOM sized particles by gravel–cobble bedload transport, hillslope connectivity, drainage density, and biological consumption. CPOM loads were 3.6 and 3.2 t/yr for the two Rocky Mountain streams. Adjusted to reflect decadal averages, values increased to 11.3 and 10.2 t/yr. CPOM yields related to the entire watershed were 2.7 and 4 kg/ha/yr for the years studied, but both streams exported similar amounts of 6.5 and 6.6 kg/ha/yr when taking the forested portion of the watershed into account. To reflect decadal averages, CPOM yields per basin area were adjusted to 8.6 and 12.6 kg/ha/yr and to 21 kg/ha/yr for the forested watershed parts. CPOM yield may be more meaningfully characterized if annual CPOM loads are normalized by the area of a seam along the stream banks together with the stream surface area rather than by the forested or total watershed area.

Spatial patterns in CO2 evasion from the global river network

AbstractCO2 evasion from rivers (FCO2) is an important component of the global carbon budget. Here we present the first global maps of CO2 partial pressures (pCO2) in rivers of stream orders 3 and higher and the resulting FCO2 at 0.5° resolution constructed with a statistical model. A geographic information system based approach is used to derive a pCO2 prediction function trained on data from 1182 sampling locations. While data from Asia and Africa are scarce and the training data set is dominated by sampling locations from the Americas, Europe, and Australia, the sampling locations cover the full spectrum from high to low latitudes. The predictors of pCO2 are net primary production, population density, and slope gradient within the river catchment as well as mean air temperature at the sampling location (r2 = 0.47). The predicted pCO2 map was then combined with spatially explicit estimates of stream surface area Ariver and gas exchange velocity k calculated from published empirical equations and data sets to derive the FCO2 map. Using Monte Carlo simulations, we assessed the uncertainties of our estimates. At the global scale, we estimate an average river pCO2 of 2400 (2019–2826) µatm and a FCO2 of 650 (483–846) Tg C yr−1 (5th and 95th percentiles of confidence interval). Our global CO2 evasion is substantially lower than the recent estimate of 1800 Tg C yr−1 although the training set of pCO2 is very similar in both studies, mainly due to lower tropical pCO2 estimates in the present study. Our maps reveal strong latitudinal gradients in pCO2, Ariver, and FCO2. The zone between 10°N and 10°S contributes about half of the global CO2 evasion. Collection of pCO2 data in this zone, in particular, for African and Southeast Asian rivers is a high priority to reduce uncertainty on FCO2.

Emissions of carbon dioxide and methane from a headwater stream network of interior Alaska

Boreal ecosystems store significant quantities of organic carbon (C) that may be vulnerable to degradation as a result of a warming climate. Despite their limited coverage on the landscape, streams play a significant role in the processing, gaseous emission, and downstream export of C, and small streams are thought to be particularly important because of their close connection with the surrounding landscape. However, ecosystem carbon studies do not commonly incorporate the role of the aquatic conduit. We measured carbon dioxide (CO2) and methane (CH4) concentrations and emissions in a headwater stream network of interior Alaska underlain by permafrost to assess the potential role of stream gas emissions in the regional carbon balance. First‐order streams exhibited the greatest variability in fluxes of CO2 and CH4, and the greatest mean pCO2. High‐resolution time series of stream pCO2 and discharge at two locations on one first‐order stream showed opposing pCO2 responses to storm events, indicating the importance of hydrologic flowpaths connecting CO2‐rich soils with surface waters. Repeated longitudinal surveys on the stream showed consistent areas of elevated pCO2 and pCH4, indicative of discrete hydrologic flowpaths delivering soil water and groundwater having varying chemistry. Up‐scaled basin estimates of stream gas emissions suggest that streams may contribute significantly to catchment‐wide CH4 emissions. Overall, our results indicate that while stream‐specific gas emission rates are disproportionately high relative to the terrestrial landscape, both stream surface area and catchment normalized emission rates were lower than those documented for the Yukon River Basin as a whole. This may be due to limitations of C sources and/or C transport to surface waters.

Carbon dioxide and methane emissions from the Yukon River system

Carbon dioxide (CO2) and methane (CH4) emissions are important, but poorly quantified, components of riverine carbon (C) budgets. This is largely because the data needed for gas flux calculations are sparse and are spatially and temporally variable. Additionally, the importance of C gas emissions relative to lateral C exports is not well known because gaseous and aqueous fluxes are not commonly measured on the same rivers. We couple measurements of aqueous CO2 and CH4 partial pressures (pCO2, pCH4) and flux across the water‐air interface with gas transfer models to calculate subbasin distributions of gas flux density. We then combine those flux densities with remote and direct observations of stream and river water surface area and ice duration, to calculate C gas emissions from flowing waters throughout the Yukon River basin. CO2 emissions were 7.68 Tg C yr−1 (95% CI: 5.84 −10.46), averaging 750 g C m−2 yr−1 normalized to water surface area, and 9.0 g C m−2 yr−1 normalized to river basin area. River CH4 emissions totaled 55 Gg C yr−1 or 0.7% of the total mass of C emitted as CO2 plus CH4 and ∼6.4% of their combined radiative forcing. When combined with lateral inorganic plus organic C exports to below head of tide, C gas emissions comprised 50% of total C exported by the Yukon River and its tributaries. River CO2 and CH4 derive from multiple sources, including groundwater, surface water runoff, carbonate equilibrium reactions, and benthic and water column microbial processing of organic C. The exact role of each of these processes is not yet quantified in the overall river C budget.

FTABLE Generation Method Effects on Instream Fecal Bacteria Concentrations Simulated With HSPF1

Abstract: Computer simulation models are used extensively for the development of total maximum daily loads (TMDLs). Specifically, the Hydrological Simulation Program‐FORTRAN (HSPF) is used in Virginia for the development of TMDLs for bacteria impairments. HSPF estimates discharge from a reach using function tables (FTABLES). The FTABLE relates stream stage, surface area, and volume to discharge from a reach. In this study, five FTABLE estimation methods were assessed by comparing their effect on various simulation outputs. Four “field‐based” methods used detailed cross‐sectional data collected via site surveys. A fifth “digital‐based” method used digital elevation data in combination with the Natural Resources Conservation Service Regional Hydraulic Geometry Curves. Sets of FTABLEs created using each method were used in simulations of instream bacteria concentration for a Virginia watershed. Several statistics relating to instream bacteria including long‐term average concentration, die‐off, and the violation rate of Virginia’s bacteria criterion were compared. The pair‐wise Student’s t‐test was used for the comparison. The HSPF simulations that used FTABLES estimated from digitally based data consistently produced significantly higher long‐term average instream fecal bacteria concentrations, significantly lower instream fecal bacteria die‐off, which is related to differences in residence time in the streams, and significantly higher water quality criterion violation rates.

COMPARISON OF HSPF OUTPUTS USING FTABLES GENERATED WITH FIELD SURVEY AND DIGITAL DATA1

ABSTRACT: The Hydrological Simulation Program‐FORTRAN (HSPF) describes discharge from a stream reach based on function tables (FTABLES) that relate stream stage, surface area, volume, and discharge. For this study, five FTABLE scenarios were compared to assess their effect on daily discharge rates predicted using HSPF. Four “field‐based” FTABLE scenarios were developed using detailed cross section surveys collected at predefined intervals along 14 reaches in the study watershed. A fifth “digital‐based” scenario was developed using digital elevation models (DEMs) and Natural Resource Conservation Service (NRCS) Regional Hydraulic Geometry Curves. The Smirnov k‐sample test was used to compare average daily discharge rates simulated with HSPF using the five FTABLE scenarios. No significant difference in simulated stream discharge was found (p = 0.99) between the five FTABLE scenarios. Additional examination of the four field‐based scenarios revealed that the number of cross sections per stream reach used to generate FTABLES had little effect on the resulting stage discharge relationship. These findings suggest that FTABLES generated using digital data are a viable option when simulating stream discharge with HSPF and that if field data are used to generate FTABLES, using fewer cross sections will not adversely affect simulated discharge predictions.

STRUCTURE AND FUNCTION OF BENTHIC ALGAL COMMUNITIES IN AN EXTREMELY ACID RIVER1

The composition of algal species and pigments and the structural and functional characteristics of the algal community were investigated in an acid stream of southwestern Spain, the Río Tinto. The algal community had low diversity and showed few seasonal differences. It was mainly made up of Klebsormidium flaccidum Kütz. (Silva, Mattox & Blackwell) that produced long greenish or purplish filaments, Pinnularia acoricola Hust. (producing brown patches) and Euglena mutabilis Schmitz. The algal filaments made up a consistent biofilm that also included fungal hyphae, iron bacterial sheaths, diatoms, and mineral particles. HPLC analyses on Río Tinto samples showed that undegraded chl accounted for 67% of the total chl in the filamentous patches but were a minority in the brown patch (2.6%). The brown patch had a concentration of carotenoids eight times lower than that observed in the green patch. When chl concentrations were weighted for the proportion of the different patches on the streambed, undegraded chl a accounted for 89.2 mg chl a·m−2 of stream surface area (5.4 g C·m−2). This high algal biomass was supported by relatively high nutrient concentrations and by a high phosphatase activity (Vmax = 137.7 nmol methylumbelliferyl substrate·cm−2·h−1, Km = 0.0045 μM). The remarkable algal biomass in Río Tinto potentially contributed to the bacterial–fungal community and to the macroinvertebrate community and emphasizes the role that the algae may have in the organic matter cycling and energy flow in extreme systems dominated by heterotrophic microorganisms.

The Use of Streambed Temperature Profiles to Estimate the Depth, Duration, and Rate of Percolation Beneath Arroyos

Temporal variations in a streambed temperature profile between 30 and 300 cm beneath Tijeras Arroyo, New Mexico, were analyzed at 30‐min intervals for 1990 to estimate the depth, duration, and rate of percolation during streamflows. The depth of percolation was clearly documented by the rapid response of the streambed temperature profile to streamflows. Results indicate that the streambed possessed small thermal gradients with significant diurnal variations from late November to late May, indicating that ephemeral streamflows created continuous, advection‐dominated heat transport to depths below 300 cm during this period. Timing and duration of percolation suggested by temporal variations in the temperature profile were verified by comparison with measured streamflow records for the study reach over 1990. Percolation rates were estimated using a technique based on the travel time of the daily maximum temperature into the streambed. Percolation rates were compared with streambed seepage rates determined from measurements of streamflow loss, stream surface area, and stream evaporative loss for the entire study reach. Travel time estimates of streambed percolation rates ranged from 9 to 40 cm/hr, while streamflow estimates of streambed seepage rates ranged from 6 to 26 cm/hr during the study period. Discrepancies between streambed percolation and seepage rates may be caused by differences in the areal extent of measurements for percolation versus seepages rates. In summary, the depth, timing, and duration of streamflow‐induced percolation were well documented by temporal variations in a single streambed temperature profile, while rates of percolation based on the temperature profile were about double the seepage rates based on streamflow records for the entire study reach.

Interreplicate Variance and Statistical Power of Electrofishing Data from Low-Gradient Streams in the Southeastern United States

I assessed the relationship between interreplicate variance and the mean for electrofishing catch per unit effort (defined as number of fish per 100 m2 of stream surface area) from 56 sites in 22 first- through fourth-order streams on the coastal plain of South Carolina. Variance and mean were strongly and positively related. This relationship was unaffected by type and quantity of instream structure and was similar among schooling and nonschooling species. Interreplicate variance was unaffected by replicate reach lengths ranging from 60 to 130 m. Replicate lengths near the lower end of this range required less total effort (number of replicates times replicate length) than longer replicates for comparable precision. Statistical power computations indicated that large numbers of samples (>25) were required to detect small (<20%) differences among means, and that the required number of samples was greater when catch per unit effort was low, indicating that power can be increased if multiple passes are used to increase the catch. However, it required less total time to use a relatively large number of single-pass replicates than a smaller number of multiple-pass replicates to achieve a given level of precision. Investigation of several types of data transformations indicated that the log transformation was most efficient at reducing the correlation between the variance and the mean, as required for parametric statistical analysis.

Size-Specific Interactions among Larvae of the Plethodontid Salamanders Gyrinophilus porphyriticus and Eurycea cirrigera

Size-Specific Interactions among Larvae of the Plethodontid Salamanders Gyrinophilus porphyriticus and Eurycea cirrigera

Relationships between Fish Assemblage Structure and Stream Order in South Carolina Coastal Plain Streams

Forty-seven sample sites were electrofished in 22 streams on the South Carolina coastal plain. Average species numbers adjusted to a constant stream surface area were 12.7, 17.5, 21.4, and 22.0 in first- through fourth-order streams, respectively. Species addition and replacement led to large changes in species composition among stream orders. Relatively small fishes, most of which were generalized insectivores, numerically dominated headwater (first- and second-order) streams. Relatively large fishes, many of which were piscivores or benthic insectivores, were most common in fourth-order streams. Headwater species richness was higher and longitudinal species replacement was greater than often observed in other geographic regions of the United States. A comparative assessment of long-term temperature and precipitation records suggested that high species richness at headwater sites was related to mild climate and lack of steep elevation gradients. The presence of numerous small headwater species created the potential for multiple species replacements as downstream increases in habitat volume permitted the establishment of larger fish with predatory and competitive advantages. Because they support many species uncommon in larger streams, headwater streams in the southeastern coastal plain contribute importantly to biodiversity.