Hyporheic Temperature Research Articles

Thermal insulation versus capacitance: A simulation experiment comparing effects of shade and hyporheic exchange on daily and seasonal stream temperature cycles

AbstractIn streams where water temperatures stress native biota, management of riparian shade or hyporheic exchange are both considered viable management strategies for reducing the peaks of daily and seasonal stream channel temperature cycles. Although shade and hyporheic exchange may have similar effects on stream temperatures, their mechanisms differ. Improved understanding of the heat‐exchange mechanisms influenced by shade and hyporheic exchange will aid in the appropriate application of either stream temperature management strategy. To illustrate a conceptual model highlighting shade as ‘thermal insulation’ and hyporheic exchange imparting ‘thermal capacitance’ to a stream reach, we conducted an in‐silico simulation modelling experiment increasing shade or hyporheic exchange parameters on an idealized, hypothetical stream. We assessed the potential effects of increasing shade or hyporheic exchange on a stream reach using an established process‐based heat‐energy budget model of stream‐atmosphere heat exchange and incorporated an advection‐driven hyporheic heat exchange routine. The model tracked heat transport through the hyporheic zone and exchange with the stream channel, while including the effects of hyporheic water age distribution on upwelling hyporheic temperatures. Results showed that shade and hyporheic exchange similarly damped diurnal temperature cycles and differentially altered seasonal cycles of our theoretical stream. In winter, hyporheic exchange warmed simulated channel temperatures whereas shade had little effect. In summer, both shade and hyporheic exchange cooled channel temperatures, though the effects of shade were more pronounced. Our simple‐to‐grasp analogies of ‘thermal insulation’ for shade effects and ‘thermal capacitance’ for hyporheic exchange effects on stream temperature encourage more accurate conceptualization of complex, dynamic heat exchange processes among the atmosphere, stream channel, and alluvial aquifer.

Hyporheic Oxygen Dynamics in the East River, Colorado: Insights From an In‐Situ, High Frequency Time Series During Two Distinct Flow Seasons

AbstractDissolved oxygen (DO) is critical for aquatic ecosystems, however, few studies have focused on the long‐term DO dynamics in hyporheic zones, which are a function of both transport (hydrologic exchange between river and hyporheic zone) and uptake by biogeochemical reactions or respiration. We explore the dynamics of temperature and DO at 10, 20, and 35 cm depth beneath the East River, Colorado, from July–October 2017 (relatively normal water year) and April to October 2018 (comparatively low flow year), enabled by distinctive, in‐situ, high frequency (Δt = 5 min) sensors that provided continuous time‐series from the undisturbed study site over 14 months. We expect that hyporheic DO, which has a regular daily fluctuation pattern, is supplied by the surface water (at all times we estimate downwelling) and that diurnal hyporheic DO temporal patterns should be aligned with diurnal hyporheic temperature patterns. However, this was not found to be the case. Hyporheic DO becomes depleted briefly at 20 and 35 cm depths in 2017, and at all three hyporheic depths for extended periods in 2018. Whereas diurnal temperature fluctuations have consistent timings of maxima and minima, hyporheic DO rarely has as regular a pattern, and daily ranges are inconsistent. Rainfall events caused some of these changes to diurnal hyporheic DO patterns without repeatable effects. Antecedent snowpack conditions influence streamflow dynamics and therefore hyporheic DO dynamics in this alpine river. These results also point to the strong and variable influence of hyporheic microbial communities regulating hyporheic DO.

Characterizing the Flow Regime in Brook Trout Incubation Habitats and Implications for Management in a Hydro‐Regulated River

AbstractThe operation of dams, diversions, and power generation facilities unavoidably influence the ecological function of rivers. We evaluated vertical and horizontal hydraulic gradients and subsequent water temperature changes in the hyporheic zone of a spawning area for Brook Trout Salvelinus fontinalis in a managed Lake Superior tributary in Ontario, then conducted a laboratory‐based experiment to show the impact of cold shock during incubation on the timing of larval emergence and survival. Upwelling groundwater was observed at the spawning area during the monitoring period (October 28, 2016, to January 11, 2017); hyporheic water temperatures remained above 3.7°C, whereas ambient river temperature above the substrate fell to 1.5°C until water was released from the upstream control dam. After the release event, water level increased by 0.9 m over the spawning area, and within 24 h, vertical and horizontal flow gradients were reversed in the hyporheic zone; downwelling conditions were observed for 30 h between surface and hyporheic water 1.8 m below substrate, and hyporheic water temperature decreased in unison. Hyporheic temperatures at shallow inshore sites fell below 1°C for more than 53 h. In the laboratory, applying a worst‐case scenario of dam‐induced cold shock on Brook Trout redds using historic monitoring records (i.e., <1.0°C for 40 h), we found no appreciable effect on survival and development timing of incubating Brook Trout eggs compared with a control group. Survival from fertilization to hatching was high for both treatment and control groups (>90%) and lower from fertilization to emergence (55%). Overall, our results suggest that standard winter operating procedures on a Lake Superior tributary had little impact on Brook Trout egg survival and development time to hatching and emergence; however, our findings also suggest that staged winter discharges would lessen the reversal of flow in the hyporheic zone and the magnitude of temperature changes experienced at Brook Trout redds.

Invertebrate and Microbial Response to Hyporheic Restoration of an Urban Stream

All cities face complex challenges managing urban stormwater while also protecting urban water bodies. Green stormwater infrastructure and process-based restoration offer alternative strategies that prioritize watershed connectivity. We report on a new urban floodplain restoration technique being tested in the City of Seattle, USA: an engineered hyporheic zone. The hyporheic zone has long been an overlooked component in floodplain restoration. Yet this subsurface area offers enormous potential for stormwater amelioration and is a critical component of healthy streams. From 2014 to 2017, we measured hyporheic temperature, nutrients, and microbial and invertebrate communities at three paired stream reaches with and without hyporheic restoration. At two of the three pairs, water temperature was significantly lower at the restored reach, while dissolved organic carbon and microbial metabolism were higher. Hyporheic invertebrate density and taxa richness were significantly higher across all three restored reaches. These are some of the first quantified responses of hyporheic biological communities to restoration. Our results complement earlier reports of enhanced hydrologic and chemical functioning of the engineered hyporheic zone. Together, this research demonstrates that incorporation of hyporheic design elements in floodplain restoration can enhance temperature moderation, habitat diversity, contaminant filtration, and the biological health of urban streams.

Coupled Long‐Term Simulation of Reach‐Scale Water and Heat Fluxes Across the River‐Groundwater Interface for Retrieving Hyporheic Residence Times and Temperature Dynamics

AbstractFlow patterns in conjunction with seasonal and diurnal temperature variations control ecological and biogeochemical conditions in hyporheic sediments. In particular, hyporheic temperatures have a great impact on many temperature‐sensitive microbial processes. In this study, we used 3‐D coupled water flow and heat transport simulations applying the HydroGeoSphere code in combination with high‐resolution observations of hydraulic heads and temperatures to quantify reach‐scale water and heat flux across the river‐groundwater interface and hyporheic temperature dynamics of a lowland gravel bed river. The model was calibrated in order to constrain estimates of the most sensitive model parameters. The magnitude and variations of the simulated temperatures matched the observed ones, with an average mean absolute error of 0.7°C and an average Nash Sutcliffe efficiency of 0.87. Our results indicate that nonsubmerged streambed structures such as gravel bars cause substantial thermal heterogeneity within the saturated sediment at the reach scale. Individual hyporheic flow path temperatures strongly depend on the flow path residence time, flow path depth, river, and groundwater temperature. Variations in individual hyporheic flow path temperatures were up to 7.9°C, significantly higher than the daily average (2.8°C), but still lower than the average seasonal hyporheic temperature difference (19.2°C). The distribution between flow path temperatures and residence times follows a power law relationship with exponent of about 0.37. Based on this empirical relation, we further estimated the influence of hyporheic flow path residence time and temperature on oxygen consumption which was found to partly increase by up to 29% in simulations.

Hydrological and Geophysical Investigation of Streamflow Losses and Restoration Strategies in an Abandoned Mine Lands Setting

Abstract Longitudinal discharge and water-quality campaigns (seepage runs) were combined with surface geophysical surveys, hyporheic zone temperature profiling, and watershed-scale hydrological monitoring to evaluate the locations, magnitude, and impact of stream-water losses from the West Creek sub-basin of the West West Branch Schuylkill River into the underground Oak Hill Mine complex that extends beneath the watershed divide. Abandoned mine drainage, containing iron and other contaminants, from the Oak Hill Boreholes to the West Branch Schuylkill River was sustained during low-flow conditions and correlated to streamflow lost through the West Creek streambed. During high-flow conditions, streamflow was transmitted throughout West Creek; however, during low-flow conditions, all streamflow from the perennial headwaters was lost within the 300 to 600 m “upper reach,” where an 1889 mine map indicated steeply dipping coalbeds underlie the channel. During low-flow conditions, the channel within the “intermediate reach,” 700 to 1,650 m downstream, gained groundwater seepage with higher pH and specific conductance than upstream; however, all streamflow 1,650 to 2,050 m downstream was lost to underlying mines. Electrical resistivity and electromagnetic conductivity surveys indicated conductive zones beneath the upper reach, where flow loss occurred, and through the intermediate reach, where gains and losses occurred. Temperature probes at 0.06 to 0.10 m depth within the hyporheic zone of the intermediate reach indicated potential downward fluxes as high as 2.1 × 10−5 m/s. Cumulative streamflow lost from West Creek during seepage runs averaged 53.4 L/s, which equates to 19.3 percent of the daily average discharge of abandoned mine drainage from the Oak Hill Boreholes and a downward flux of 1.70×10−5 m/s across the 2.1 km by 1.5 m West Creek stream-channel area.

English

Abstract Relatively few studies have examined fine-scale habitat use of crayfish in headwater streams, despite increasing awareness of the importance of such habitats. Orconectes meeki meeki, Meek’s crayfish, is endemic to the upper White River drainage of Missouri and Arkansas, and Orconectes williamsi, Williams’ crayfish, occurs only there and in the Arkansas River drainage. Our objective was to describe speciesspecific habitat use of these crayfishes at two spatial scales (pool-riffle and microhabitat) within a seasonally intermittent stream. From May through August 2008, we sampled ten riffles and five pools in Rock Creek, Missouri, to quantify surface and hyporheic environmental variables. Density of O. m. meeki was similar between riffles and pools; O. williamsi density was greater in riffles. At the riffle scale, O. m. meeki was positively associated with wetted channel width and upwelling hyporheic zones; O. williamsi was negatively associated with surface and hyporheic water temperatures. At the microhabitat scale within riffles, O. m. meeki was positively associated with wetted depth and pebble-cobble substrates; O. williamsi was positively associated with surface velocity and pebble-cobble substrates. Habitat use was relatively static for both species as surface flows waned from May through August. Our research provides detailed, fine-scale habitat associations of the crayfish to complement existing knowledge of these species at coarser spatial scales.

Hyporheic temperature dynamics and heat exchange near channel‐spanning logs

The flow of river water around large woody debris (LWD) creates pressure gradients along the riverbed that drive river‐groundwater mixing, or hyporheic exchange, and heat transport within the hyporheic zone. We quantify hyporheic fluid and heat exchange induced by current interaction with channel‐spanning logs using two approaches: laboratory flume experiments and numerical simulations that link turbulent open‐channel fluid flow, porous fluid flow, and heat transport. Flume and numerical experiments show that logs produce a characteristic diel temperature pattern within sediment that shifts with log blockage ratio (a fraction of the channel depth blocked by the log), channel Froude number, and sediment permeability. Upstream of a log, downwelling water transports the river's diel thermal signal deep into the sediment. Downstream, upwelling water from shallow flow paths has a thermal signal similar to the surface water, while upwelling water from deep flow paths forms a wedge of buffered (low‐amplitude) temperatures. Since most hyporheic water emerges from shallow flow paths, upwelling water has limited potential to buffer surface water temperature. Because of historic channel clearing practices, modern rivers have unnaturally low densities of LWD. A key implication is that LWD removal has contributed to thermal homogenization and potential degradation of hyporheic habitats. LWD reintroduction is a promising strategy to improve vertical connectivity in rivers and increase thermal patchiness within the hyporheic zone. However, hyporheic exchange near LWD may not impact diel surface water temperatures at the reach scale.

Seasonal hyporheic temperature dynamics over riffle bedforms

AbstractThere is growing interest in riverbed temperature due to the ecological and biogeochemical significance of the hyporheic zone, and its potential to moderate river temperature. Riffles exhibit complex thermal behaviour, hypothesized to be caused by local alteration of groundwater–surface water (GW–SW) interactions, but hitherto most research has been in upland/gravel‐bed/hard rock catchments. Accordingly, this article aims: (1) to characterize spatio‐temporal variability in hyporheic temperature over two riffles (R1 and R2) in a lowland river basin (Tern, Shropshire, UK) underlain by sandstone, and (2) to explain thermal dynamics by inferring hyporheic processes and the influence of GW–SW interactions. Hyporheic (riffle head, crest and tail at 0·1, 0·2 and 0·4 m), water column, spring water and air temperature were collected at 15 min resolution over 22 months and used to explore seasonal variations. Borehole water levels and temperature provide insight into groundwater variability over a hydrological year. Hyporheic temperature is cooler (warmer) than water column in summer (winter), with convergence in spring and autumn. Riffle heads and R2 crest yield small thermal gradients, and R1 tail larger vertical difference. R1 crest temperature is similar and attenuated (cf. water column) at all depths. R2 tail temperature differs markedly from surface water. Thus, hyporheic temperature varies temporally across and between riffles, reflecting: (1) hydroclimatological controls on river and groundwater temperature, and (2) hydrological, local morphological and sedimentary controls on surface water and groundwater flux. This research demonstrates the utility of depth‐related riverbed temperature time‐series in understanding hyporheic zone processes and GW–SW interactions. Copyright © 2009 John Wiley & Sons, Ltd.

Hyporheic exchange flow around constructed in‐channel structures and implications for restoration design

AbstractIn‐channel rock vane structures are widely used in stream restoration as a way to reduce stream channel erosion and create pool or riffle features. When these structures change hydraulic gradients they may affect ecological stream functions, such as hyporheic exchange flow (HEF) patterns. A study of constructed in‐channel structure controls on HEF was conducted in the third‐order Batavia Kill, New York using stream and hyporheic temperature amplitude analysis and computational fluid dynamics (CFD) hydraulic simulations. Temperature monitors were installed in the water column and channel bed at six locations around each of seven in‐channel restoration structures (three cross‐vanes and four J‐hooks) at baseflow in 2007. Elevation surveys of the structures were then used to simulate HEF using CFD. The results indicate a pattern of pronounced upwelling in the run section just below the structure, upwelling transitioning to downwelling within the pool, and pronounced downwelling in the glide out of the pool. This pattern is consistent with natural riffle pool sequences. The direction of HEF inferred from the temperature amplitude analysis agreed with the direction of flow simulated with CFD at 80% of the locations, and the few disagreements were expected due to model limitations. CFD simulation demonstrated that increasing stream flows result in changes in HEF spatial patterns and magnitude at each structure. This work illustrates how CFD simulations can guide design of in‐channel restoration structures for HEF function. Copyright © 2009 John Wiley & Sons, Ltd.

Do stream macroinvertebrates use instream refugia in response to severe short‐term flow reduction in New Zealand streams?

Summary1. Demand for water is increasing and water managers need to know how much they can remove from a stream before there are significant detrimental effects on its biological integrity. Flow reduction alters a number of habitat variables known to be important to aquatic invertebrates such as depth, velocity, temperature and fine sediment accumulation. Some taxa may attempt to use instream refugia to mitigate the effects of flow reduction.2. We experimentally manipulated flows by constructing weirs and diversions in three small New Zealand streams. Discharge was reduced by 88–96%. We tested the hypothesis that macroinvertebrates would use pools and the hyporheic zone as refugia during short‐term (1‐month) periods of reduced flow.3. We sampled hyporheic invertebrates with colonization chambers and pool invertebrates with kick nets within a before‐after, control‐impact (BACI) experimental design. A suite of physicochemical parameters was measured concurrently including surface and hyporheic temperatures.4. Flow reduction significantly decreased velocity (60–69%) in all streams. Depth (18–61%) and wetted width (24–31%) tended to decrease but these changes were not always significant. Sediment cover increased the most in farmland streams (10–80%). Apart from decreasing temperature range (18–26%), flow reduction had little impact on the surface water temperatures.5. Flow reduction had no impact on the abundance of common pool macroinvertebrates or on the abundance, vertical distribution or community composition of hyporheic macroinvertebrates.6. Our results suggest that aquatic macroinvertebrates are resistant to short‐term, severe flow reduction as long as some water remains.

Influence of hyporheic flow and geomorphology on temperature of a large, gravel‐bed river, Clackamas River, Oregon, USA

AbstractThe hyporheic zone influences the thermal regime of rivers, buffering temperature by storing and releasing heat over a range of timescales. We examined the relationship between hyporheic exchange and temperature along a 24‐km reach of the lower Clackamas River, a large gravel‐bed river in northwestern Oregon (median discharge = 75·7 m3/s; minimum mean monthly discharge = 22·7 m3/s in August 2006). With a simple mixing model, we estimated how much hyporheic exchange cools the river during hot summer months. Hyporheic exchange was primarily identified by temperature anomalies, which are patches of water that demonstrate at least a 1 °C temperature difference from the main channel. Forty hyporheic temperature anomalies were identified through field investigations and thermal‐infrared‐radiometry (TIR) in summer 2006. The location of anomalies was associated with specific geomorphic features, primarily bar channels and bar heads that act as preferential pathways for hyporheic flow. Detailed field characterization and groundwater modelling on three Clackamas gravel bars indicate residence times of hyporheic water can vary from hours to weeks and months. This was largely determined by hydraulic conductivity, which is affected by how recently the gravel bar formed or was reworked. Upscaling of modelled discharges and hydrologic parameters from these bars to other anomalies on the Clackamas network shows that hyporheic discharge from anomalies comprises a small fraction (≪1%) of mainstem discharge, resulting in small river‐cooling effects (0·012 °C). However, the presence of cooler patches of water within rivers can act as thermal refugia for fish and other aquatic organisms, making the creation or enhancement of hyporheic exchange an attractive method in restoring the thermal regime of rivers. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of River Level on Temperature and Hydraulic Gradients in Chum and Fall Chinook Salmon Spawning Areas Downstream of Bonneville Dam, Columbia River

AbstractChum salmon Oncorhynchus keta and fall Chinook salmon O. tshawytscha segregate spatially during spawning in the Ives Island side channel of the lower Columbia River downstream from Bonneville Dam. Previous research during one spawning season (2000) suggested that these species selected spawning habitats based on differences in hyporheic temperature and vertical hydraulic gradient (VHG). In this study we confirmed the spatial segregation of spawning based on hyporheic characteristics over 4 years (2001–2004) and examined the effects of load‐following operations (power generation to meet short‐term electrical demand) at Bonneville Dam on hyporheic function and characteristics. We found that during the study period hyporheic temperature and VHG in chum salmon spawning areas were highly variable during periods of load‐following operation, when river levels fluctuated. In contrast, hyporheic water temperature and VHG within chum salmon spawning areas fluctuated less when river levels were not changing owing to load‐following operation. Variable temperature and VHG could affect chum and fall Chinook salmon spawning segregation and incubation success by altering the cues each species uses to select redd sites. Additional research will be required to fully assess the effects of load‐following operations on the hyporheic environment, spawning site selection, and incubation success of chum and fall Chinook salmon downstream from Bonneville Dam.

Water quality changes in hyporheic flow paths between a large gravel bed river and off-channel alcoves in Oregon, USA

Changes in water quality that occur as water flows along hyporheic flow paths may have important effects on surface water quality and aquatic habitat, yet very few studies have examined these hyporheic processes along large gravel bed rivers. To determine water quality changes associated with hyporheic flow along the Willamette River, Oregon, we studied hyporheic flow at six-bar deposit sites positioned between the main river channel and connected lentic alcoves. We installed piezometers and wells at each site and measured water levels and water quality in river, hyporheic and alcove water. Piezometric surfaces along with substrate characteristics were used to determine hyporheic flow path direction and hyporheic flow rate. At all sites, hyporheic flow moved from the river through bar deposits into alcove surface water. Stable isotope analysis showed little influence of upwelling groundwater. At a majority of sites, hyporheic dissolved oxygen and ammonium decreased relative to river water, and hyporheic specific conductance, nitrate and soluble reactive phosphorous increased relative to river water. At three sites, hyporheic temperature decreased 3–7°C relative to river water; there was less temperature change at the other three sites. At the two sites with the highest hyporheic flow rates, hyporheic cooling was propagated into the alcove surface water. Hyporheic changes had the greatest effect on alcove water quality at sites with highly permeable substrates and high-hyporheic flow rates. The best approach to enhancing hyporheic flows and associated water quality functions is through restoring fluviogeomorphic channel processes that create and maintain high-permeability gravel deposits conducive to hyporheic flow. Copyright © 2006 John Wiley & Sons, Ltd.

A GIS model of subsurface water potential for aquatic resource inventory, assessment, and environmental management.

Biological, chemical, and physical attributes of aquatic ecosystems are often strongly influenced by groundwater sources. Nonetheless, widespread access to predictions of subsurface contributions to rivers, lakes, and wetlands at a scale useful to environmental managers is generally lacking. In this paper, we describe a "neighborhood analysis" approach for estimating topographic constraints on spatial patterns of recharge and discharge and discuss how this index has proven useful in research, management, and conservation contexts. The Michigan Rivers Inventory subsurface flux model (MRI-DARCY) used digital elevation and hydraulic conductivity inferred from mapped surficial geology to estimate spatial pattems of hydraulic potential. Model predictions were calculated in units of specific discharge (meters per day) for a 30-m-cell raster map and interpreted as an index of potential subsurface water flux (shallow groundwater and event through-flow). The model was evaluated by comparison with measurements of groundwater-related attributes at watershed, stream segment, and local spatial scales throughout Lower Michigan (USA). Map-based predictions using MRI-DARCY accounted for 85% of the observed variation in base flow from 128 USGS gauges, 69% of the observed variation in discharge accrual from 48 river segments, and 29% of the residual variation in local groundwater flux from 33 locations as measured by hyporheic temperature profiles after factoring out the effects of climate. Although it does not incorporate any information about the actual water table surface, by quantifying spatial variation of key constraints on groundwater-related attributes, the model provides strata for more intensive study, as well as a useful spatial tool for regional and local conservation planning, fisheries management, wetland characterization, and stream assessment.

Thermal heterogeneity in the hyporheic zone of a glacial floodplain

We examined the thermal regime of surface and hyporheic waters at three kryal sites and four krenal streams within the channel network of a glacial floodplain. Temperature was continuously measured for 1 year in the surface stream and at sediment depths of 30 and 80 cm. The vertical pattern of water temperature was strongly influenced by the direction and intensity of surface water – groundwater exchanges. At sites characterized by strong downwelling of surface waters, the thermal regimes of surface and hyporheic waters were virtually identical. In contrast, inputs of groundwater substantially increased mean summer temperatures in the hyporheic zone of the main kryal channel, decreased summer temperatures in the hyporheic zone of krenal streams, and elevated hyporheic temperatures of all stream types during winter. Groundwater from different sources had dramatically different effects on the seasonal regime of temperature in the hyporheic zone. Inflow of shallow alluvial groundwater had minimal effects on seasonal patterns of hyporheic temperature, whereas upwelling from deep alluvial and hillslope aquifers resulted in significant time lags and differences in seasonal amplitudes between surface and hyporheic temperatures. The unexpectedly high thermal heterogeneity of hyporheic waters presumably sustains biodiversity and stimulates ecosystem processes in this glacial floodplain.

Thermal heterogeneity in the hyporheic zone of a glacial floodplain

We examined the thermal regime of surface and hyporheic waters at three kryal sites and four krenal streams within the channel network of a glacial floodplain. Temperature was continuously measured for 1 year in the surface stream and at sediment depths of 30 and 80 cm. The vertical pattern of water temperature was strongly influenced by the direction and intensity of surface water – groundwater exchanges. At sites characterized by strong downwelling of surface waters, the thermal regimes of surface and hyporheic waters were virtually identical. In contrast, inputs of groundwater substantially increased mean summer temperatures in the hyporheic zone of the main kryal channel, decreased summer temperatures in the hyporheic zone of krenal streams, and elevated hyporheic temperatures of all stream types during winter. Groundwater from different sources had dramatically different effects on the seasonal regime of temperature in the hyporheic zone. Inflow of shallow alluvial groundwater had minimal effects on ...

Land‐use effects on the hyporheic ecology of five small streams near Hamilton, New Zealand

Although the importance of the subsurface saturated interstitial zone (hyporheic zone) to the ecological functioning and maintenance of water quality of stream ecosystems is well known, there is little information on the impacts of different forms of land use upon this zone. Hyporheic physico‐chemistry and invertebrates were compared among small streams draining hill‐country catchments under pasture, exotic pine forest, and native forest near Hamilton, New Zealand. In streams draining native forest, the hyporheic zone harboured a relatively diverse invertebrate fauna comprising mostly taxa common in the surface benthos, although a few apparently obligate hyporheic taxa (ostracods, blind amphipods) were collected. Few individuals and taxa occupied the hyporheic zones of streams draining pasture with some groups such as water mites conspicuously absent. The hyporheos of the stream in exotic pine forest was similar in richness and abundance to that of the pasture streams. Hyporheic water temperatures were significantly higher in the pasture streams than those in pine or native forest. There were strong positive correlations between percentage saturation of dissolved oxygen in the hyporheic zones of all streams and both species richness and total invertebrate abundance. We suggest that land clearance for pasture leads to hill slumping and siltation that bury the lateral bars along the stream channels, rendering this habitat unsuitable for hyporheic invertebrates. Channel narrowing and incision may physically remove further hyporheic habitat, and the reduction of flushing flows to remove interstitial silt and clay leads to low hyporheic dissolved oxygen concentrations and reduced colonisation by surface benthos.

Hyporheic temperature patterns within riffles

This paper outlines the results of a pilot study using data at 12 min intervals from 20 miniature temperature dataloggers to establish vertical and longitudinal temperature patterns within a river bed. Data are presented from two adjacent riffles immediately below the Blithfield Reservoir on the River Blithe, Staffordshire, UK, for a five day period in July 1994. Hyporheic temperatures were warmer than those of groundwater, colder than those of surface water and decreased with depth into the river bed. At the heads of the riffles shallow sites (20 cm depth) mirrored surface temperature patterns but were on average 1.53°C cooler while deeper sites (40 cm depth) were 2.60°C cooler on average. Hyporheic temperatures lagged behind the surface water pattern and lag times increased with depth. Sites at the riffle tails generally displayed temperature patterns similar to those of the groundwater system: on average, temperatures were 4.32°C cooler than surface water temperatures and showed no significant variation. Hyporheic temperature patterns at the heads of the riffles suggested downwelling surface water, while the tails appeared to be influenced by upwelling groundwater. Both riffles displayed similar hyporheic temperature patterns but riffle 1 was on average 1.22°C warmer per site than riffle 2. Temperature differences between the riffles are attributed to bed form and substratum composition. These factors may have significant ecological implications including rates of organic matter decomposition, invertebrate life cycles and salmonid egg hatching times.

Seasonal biogeochemical patterns in surface water, subsurface hyporheic, and riparian ground water in a temperate stream ecosystem

Relationships among stream surface water (9-km reach), substream hyporheic water (10-m pool-riffle-pool reach), and riparian groundwater (single site) biogeochemical patterns were examined in four seasons (1989-1990) in a temperate stream ecosystem (Michigan, USA). Surface water concentrations of chloride, dissolved organic carbon (DOC), and chlorophyll-a exceeded those of groundwater during most sampling periods. Groundwater and hyporheic concentrations of silica (SiO 2 ), soluble reactive phosphorus (SRP), nitrate+nitrite (NO 3 -N), and ammonium (NH 4 -N) were generally higher than in surface water, particularly during summer. Hyporheic temperatures exhibited sharpest gradients with depth and distance downstream beneath a study riffle during summer and winter. Biologically reactive solutes (SRP, NO 3 -N, NH 4 -N, and DOC) generally decreased with depth and increased with distance beneath the riffle during summer and winter as well. Hyporheic patterns were indistinct or more variable during spring and fall and were attributed, in part, to the breakdown of thermal gradients within the bed, similar to seasonal lake turnover (mixis). Similarities between longitudinal/depth gradients in hyporheic patterns and longitudinal stream channel patterns for SiO 2 and Cl (the more conservative solutes) indicated potential cumulative groundwater discharge effects through successive hyporheic zones over stream distance. Relationships between hyporheic patterns, stream channel patterns, and the biologically reactive solutes (SRP, NO 3 -N, NH 4 -N, DOC) were less clear because of potential biotic or abiotic uptake at the sediment-water interface. Groundwater-surface water interaction within a hyporheic zone forms a complex system structured not only by hydrologic forces but also by seasonal changes in temperature and discharge.