Upstream Riffles Research Articles

The energetic consequences of habitat structure for forest stream salmonids.

Increasing habitat availability (i.e. habitat suitable for occupancy) is often assumed to elevate the abundance or production of mobile consumers; however, this relationship is often nonlinear (threshold or unimodal). Identifying the mechanisms underlying these nonlinearities is essential for predicting the ecological impacts of habitat change, yet the functional forms and ultimate causation of consumer-habitat relationships are often poorly understood. Nonlinear effects of habitat on animal abundance may manifest through physical constraints on foraging that restrict consumers from accessing their resources. Subsequent spatial incongruence between consumers and resources should lead to unimodal or saturating effects of habitat availability on consumer production if increasing the area of habitat suitable for consumer occupancy comes at the expense of habitats that generate resources. However, the shape of this relationship could be sensitive to cross-ecosystem prey subsidies, which may be unrelated to recipient habitat structure and result in more linear habitat effects on consumer production. We investigated habitat-production relationships for juveniles of stream-rearing Pacific salmon and trout (Oncorhynchus spp.), which typically forage in low-velocity pool habitats, while their prey (drifting benthic invertebrates) are produced upstream in high-velocity riffles. However, juvenile salmonids also consume subsidies of terrestrial invertebrates that may be independent of pool-riffle structure. We measured salmonid biomass production in 13 experimental enclosures each containing a downstream pool and upstream riffle, spanning a gradient of relative pool area (14%-80% pool). Increasing pool relative to riffle habitat area decreased prey abundance, leading to a nonlinear saturating effect on fish production. We then used bioenergetics model simulations to examine how the relationship between pool area and salmonid biomass is affected by varying levels of terrestrial subsidy. Simulations indicated that increasing terrestrial prey inputs linearized the effect of habitat availability on salmonid biomass, while decreasing terrestrial inputs exaggerated a "hump-shaped" effect. Our results imply that nonlinear effects of habitat availability on consumer production can arise from trade-offs between habitat suitable for consumer occupancy and habitat that generates prey. However, cross-ecosystem prey subsidies can effectively decouple this trade-off and modify consumer-habitat relationships in recipient systems.

CHANGES OF LOWER VYCHEGDA RIVERBED RELIEF IN THE COURSE OF DREDGING AND AFTER ITS FINISHING

The paper discusses the problems of a large river morphodynamics in the course of artificial channel straightening and after that, the Lower Vychegda taken as an example. There are dune bedforms (riffles) of various size and morphology in its channel. During the period of intensive dredging many riffles were completely destroyed, others became morphologically simpler and the dynamics of their periodic reformations became simpler too. Since 1991 artificial dredging almost ceased. Some of the riffles have returned to the natural state while others have kept the human induces mode of channel deformation and relatively larger depths. At downstream ends of straight channel sections the new very shallow riffles were formed, being of maximum for the Vychegda wavelength and not eroded during the low water level period. These riffles did not exist under natural conditions prior to dredging. They formed due to irreversible anthropogenic changes of channel deformation regime on upstream riffles.

Evaluation of Steelhead Passage Flows Using Hydraulic Modeling on an Unregulated Coastal California River

Passage and habitat connectivity flows for steelhead Oncorhynchus mykiss through depth sensitive natural, low gradient, critical riffle sites were investigated in the unregulated Big Sur River, California. The River2D two-dimensional hydraulic habitat model, along with quantitative passage metrics and species-specific and lifestage-specific depth criteria, were used to evaluate and compare predicted fish passage flows with flows derived by a traditional empirical critical riffle fish passage method. Passage flows were also compared with historical unimpaired natural hydrology patterns to assess the frequency and duration of suitable passage flows under the naturally variable flow regimes characteristic of Central California coastal rivers. A strong relationship (r 2 = 0.93) was observed between flows predicted by hydraulic modeling and flows identified by the empirical critical riffle method. River2D provided validation that the flows derived using the traditional critical riffle methodology provided for contiguous passable pathways of suitable hydraulic (depth and velocity) conditions through complex cobble-dominated riffle sites. Furthermore, steelhead passage flows were spatially and temporally consistent between lagoon and upstream riffles for adults, and were generally indicative of a river system in equilibrium with a naturally variable flow regime and associated intact ecological processes. An analysis of 25years of continuous flow data indicated sufficient flows for upstream passage by young-of-year and juvenile steelhead were produced between 37% and 100% and between 1% and 95% of the time, respectively. September and October are the most challenging months for natural flows to meet young-of-year and juvenile passage and habitat connectivity flows. Careful consideration of seasonal and interannual flow variability dynamics, therefore, are critical components of an effective flow management strategy for the maintenance and protection of passage and habitat connectivity flows between lagoon and upriver habitats. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Feeding modes in stream salmonid population models: is drift feeding the whole story?

Drift-feeding models are essential components of broader models that link stream habitat to salmonid populations and community dynamics. But is an additional feeding mode needed for understanding and predicting salmonid population responses to streamflow and other environmental factors? We addressed this question by applying two versions of the individual-based model inSTREAM to a field experiment in which streamflow was varied in experimental units that each contained a stream pool and the adjacent upstream riffle. The two model versions differed only in the feeding options available to fish. Both versions of inSTREAM included drift feeding; one also included a search feeding mode to represent feeding in which food availability is largely independent of streamflow, such as feeding from the benthos, or feeding from the water column or the water’s surface in low water velocities. We compared the abilities of the two model versions to fit the observed distributions of growth by individual rainbow trout (Oncorhynchus mykiss) in the field experiment. The version giving fish the daily choice between drift or search feeding better fit observations than the version in which fish fed only on drift. Values for drift and search food availability from calibration to the individual mass changes of fish in experimental units with unaltered streamflow yielded realistic distributions of individual growth when applied to experimental units in which streamflow was reduced by 80 %. These results correspond with empirical studies that show search feeding can be an important alternative to drift feeding for salmonids in some settings, and indicate that relatively simple formulations of both processes in individual-based population models can be useful in predicting the effects of environmental alterations on fish populations.

Sediment routing hypothesis for pool‐riffle maintenance

ABSTRACTThis paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool‐riffle maintenance in sinuous upland gravel‐bed streams. The findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments, if little or no sediments are fed into them. A combination of clast tracing using passive integrated transponder (PIT) tagging and bedload traps (positioned along the thalweg on the upstream riffle, pool entrance, pool exit and downstream riffle) are used to provide information on clast pathways and sediment sorting through a single pool‐riffle unit. Computational fluid dynamics (CFD) is also used to explore hydraulic variability and flow pathways. Clast tracing results provide a strong indication that clasts are not fed through pools, rather they are transported across point bar surfaces, or around bar edges (depending upon previous clast position, clast size, and event magnitude). Spatial variations in bedload transport were found throughout the pool‐riffle unit. The pool entrance bedload trap was often found to be empty, when the others had filled, further supporting the notion that little or no sediment was fed into the pool. The pool exit slope trap would occasionally fill with sediment, thought to be sourced from the eroding outer bank. CFD results demonstrate higher pool shear stresses (τ ≈ 140 N m–2) in a localized zone adjacent to an eroding outer bank, compared to the upstream and downstream riffles (τ ≈ 60 N m–2) at flows of 6 · 2 m3 s–1 (≈ 60% of the bankfull discharge) and above. There was marginal evidence for near‐bed velocity reversal. Near‐bed streamlines, produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg. Some streamlines appear to brush the outer edge of the pool for the 4 · 9 m3 s–1 to 7 · 8 m3 s–1 (between 50 and 80% of the bankfull discharge) simulations, however complete avoidance was found for discharges greater than this. Copyright © 2013 John Wiley & Sons, Ltd.

Flow in culverts as a potential mechanism of stream fragmentation for native and nonindigenous crayfish species

Habitat fragmentation and nonindigenous species are 2 of the leading causes of species loss globally. Thus, scientific information is needed to assess their interactive effects on the biota of lotic ecosystems. We tested the hypothesis that culverts (i.e., pipes) at road–stream intersections elevate flow velocities and differentially slow upstream locomotion of native relative to nonindigenous crayfishes. We: 1) mapped culvert locations and measured their flow; 2) quantified movement of Orconectes propinquus (native) in culverts with velocities of 2, 31, and 42 cm/s; 3) compared the movement of Orconectes rusticus (nonindigenous), O. propinquus, and Orconectes virilis (native) in a culvert with velocity of 30 cm/s; and 4) used a recirculating flume to determine the water velocity that impeded upstream movement (impedance velocity) of each species. Culverts had higher velocities than upstream riffles during baseflow conditions. Orconectes propinquus moved upstream in culverts with flows >30 cm/s, but individuals slipped more frequently and traveled slower than those tested at 2 cm/s. Orconectes rusticus moved upstream faster in culverts and had an impedance velocity ∼6 cm/s faster than O. propinquus (34.0 ± 1.2 cm/s, mean ± 1 SE) and ∼8 cm/s faster than O. virilis (32.0 ± 1.1 cm/s). Culverts impeded crayfish upstream movement differently among species. Nonindigenous species tolerated higher flows, so culverts may create a filter that favors the spread of invasive species. Culverts should be designed to keep water velocity <30 cm/s to mitigate flow effects on crayfish passage, thereby minimizing the possible combined effects of fragmentation and nonindigenous species introductions.

Niche Partitioning of the Sympatric Yellowcheek Darter Etheostoma moorei and Rainbow Darter Etheostoma caeruleum in the Little Red River, Arkansas

We studied how habitat selection influences the distribution and densities of a stenotypic yellowcheek darter Etheostoma moorei (Raney and Suttkus) and eurytypic rainbow darter E. caeruleum (Storer) in a headwater stream subjected to periodic drying. We spatially sampled in habitats within riffle substrates and at four sites four to six times per year in the Middle Fork of the Little Red River. Yellowcheek darters were associated with cobble and gravel more than rainbow darters. Rainbow darters moved along the substrate surface and positioned themselves further downstream in riffles, whereas yellowcheek darters occurred further upstream in or near crevices between gravel and cobble. Neither species was found in the hyporheic zone during riffle drying. The rainbow darters used pools during riffle drying but not yellowcheek darters, which could explain recolonization of previously dry upstream sites by rainbow darters but not yellowcheek darters. Rainbow darters were at their greatest densities in upstream riffles where yellowcheek darters had not recolonized, suggesting niche partitioning. These temporal differences in microhabitat selection appear to enable the coexistence of these two sympatric and ecologically similar darters.

Relations between invertebrate drift and flow velocity in sand-bed and riffle habitats and the limits imposed by substrate stability and benthic density

Downstream drift of benthic invertebrates is one of the defining characteristics of lotic environments. We collected 90 drift samples from adjacent sand-bed and riffle sections in a Mediterranean river during low flow conditions. Our goal was to compare drift fluxes (densities and instantaneous drift) and model the drift response across the velocity gradient in each habitat. Velocity distributions did not differ significantly between the habitats, but bedload transport rates and suspended sediment concentrations were significantly higher in the sand-bed habitat, where substrate was finer and less stable than in the riffle habitat. Benthic invertebrate densities differed by an order of magnitude between the habitats, with low and relatively homogeneous densities in the sand-bed. However, mean drift density did not differ significantly between the habitats. Quantile regression indicated that the upper limits of the drift response across the velocity gradient differed between habitats. Sample values below these upper limits suggest that substrate stability and benthic density act to constrain drift in some locations. Instantaneous drift was much higher in the sand-bed than in the riffle habitat, with up to 42% of the benthos present in the drift at any given moment. The taxonomic composition of benthic and drift samples suggested that animals from the riffle contributed to drift in the sand-bed habitat. The high loss rates in the sand-bed habitat suggest a rapid turnover of animals, supported by invertebrates arriving from the upstream riffle.

Backwater control on riffle–pool hydraulics, fish habitat quality, and sediment transport regime in gravel-bed rivers

Summary The importance of channel non-uniformity to natural hydrogeomorphic and ecological processes in gravel-bed rivers is becoming increasingly known, but its use in channel rehabilitation lags behind. Many projects still use methods that assume steady, uniform flow and simple channel geometries. One aspect of channel non-uniformity that has not been considered much is its role in controlling backwater conditions and thus potentially influencing patterns of physical habitat and channel stability in sequences of riffles and pools. In this study, 2D hydrodynamic models of two non-uniform pool–riffle–pool configurations were used to systematically explore the effects of four different downstream water surface elevations at three different discharges (24 total simulations) on riffle–pool ecohydraulics. Downstream water surface elevations tested included backwater, uniform, accelerating, and critical conditions, which are naturally set by downstream riffle-crest morphology but may also be re-engineered artificially. Discharges included a fish-spawning low flow, summer fish-attraction flow, and a peak snowmelt pulse. It was found that the occurrence of a significant area of high-quality fish spawning habitat at low flow depends on riffles being imposed upon by backwater conditions, which also delay the onset of full bed mobility on riffles during floods. The assumption of steady, uniform flow was found to be inappropriate for gravel-bed rivers, since their non-uniformity controls spatial patterns of habitat and sediment transport. Also, model results indicated that a “reverse domino” mechanism can explain catastrophic failure and re-organization of a sequence of riffles based on the water surface elevation response to scour on downstream riffles, which then increases scour on upstream riffles.

Use of slope creation for rehabilitating incised, regulated, gravel bed rivers

Gravel‐bedded channels often become incised and degraded below dams. Gravel can be added to the channel to rehabilitate hydrogeomorphic conditions, including those promoting salmon spawning. When implemented without increasing bed slope, gravel addition at downstream riffles back floods upstream riffles. A 2‐year gravel augmentation project was done to test the efficacy of a new method for “slope creation.” Riffle‐to‐riffle slope was raised from 0.002 to 0.008 by adding gravel to the most upstream riffle. When gravel was added to the next downstream riffle a year later, riffle‐to‐riffle slope decreased to the sought after 0.004. After the study, the area of high‐quality Chinook salmon spawning habitat increased 471%. The number of redds observed went from 62 to 161 during the study despite a 50% decline of in‐river spawners. This eliminates variations in migrant population size and hatchery take as alternative explanations. Slope creation can be a useful aid for rehabilitating regulated rivers.

Effects of Steelhead Density on Growth of Coho Salmon in a Small Coastal California Stream

Weight change in age-0 coho salmon Oncorhynchus kisutch at about natural density was negatively related to the density of juvenile steelhead (anadromous rainbow trout O. mykiss) in a 6-week experiment conducted in July–August 1993 in the north and south forks of Caspar Creek, California. The experiment used 12 enclosed stream sections, each containing a pool and a portion of upstream riffle, with two replicates of three steelhead densities—zero, natural density (1×), and twice the natural density (2×)—on both the north and south forks. The natural density of coho salmon was about one-sixth the density of steelhead. Coho salmon survival was high (87% overall) and not related to treatments. In the north fork, coho salmon weight change was positive in zero density steelhead treatments, zero in 1× treatments, and negative in 2× treatments. Coho salmon weight change in the south fork was less favorable than in the north fork but was also negatively related to the density of steelhead. These results indicate that under some conditions resource partitioning by salmonid species does not eliminate negative interspecific interactions.

The combined effects of chlorine and ammonia on litter breakdown in outdoor experimental streams

The response of Potamogeton crispus L. breakdown to controlled doses of different levels of chlorine and chlorine + ammonia was investigated over two years in outdoor experimental streams. In 1985, downstream riffles of 2 streams were dosed (observed in-stream concentrations) at ca. 10 µg/L Total Residual Chlorine (TRC), one stream at 64 µg/L TRC and one stream at 230 µg/L TRC. Two control streams were not dosed and the upstream riffles of each stream served as within stream controls. In 1986, the downstream riffle of one stream was dosed at 70 µg/L TRC and a second stream was dosed at 200 µg/L TRC. Four streams were also dosed with 2.5 mg/L NH3-N: one stream with no chlorine, one stream with ca. 10 µg/L TRC, one with 56 µg/L TRC, and one with 150 µg/L TRC. A seventh stream was dosed for 2 h at 2000 µg/L TRC and 2.5 mg/L ammonia and then allowed to recover (recovery stream). Each year, litter decomposition (degree day k values) was measured during two 35 day trials (Jun–Jul and Aug–Sep). In 1985, when streams were dosed with chlorine alone, decomposition was significantly reduced with the high (230 µg/L TRC) chlorine dose. Downstream decomposition was 27% (Jun–Jul) and 59% (Aug–Sep) of the upstream (control) rate. No other chlorine effects were found during this period. In Jun–Jul 1986, there was significantly lower decomposition in the downstream dosed sites of the 200 µg/L TRC alone stream, the 146 µg/L TRC + ammonia stream and the recovery stream; downstream decay rates were (respectively) 56%, 42% and 64% of the upstream control sites. No other up-down pairs were different in July 1986. In Aug–Sep, all three streams with chlorine + ammonia (6, 56 and 146 µg/L TRC + 2,5 mg/L ammonia) and the 70 µg/L TRC alone stream had significantly lower decomposition rates in the downstream dosed sites. For these streams, downstream decay rates ranged from 46% (high chlorine + ammonia) to 73% (low chlorine + ammonia) of the upstream control rates. No other up-down pairs were different during this trial. Up and downstream sites of the stream dosed with 2.5 mg/L ammonia alone were nearly identical for both trials (< 3% difference). These results indicate that TRC at less than 250 µg/L can significantly reduce litter decomposition and strongly suggest that addition of ammonia to chlorinated water can increase the toxic effect of chlorine.

Effects of chronic chlorine exposure on litter processing in outdoor experimental streams

SUMMARY 1. The effects of chlorine on litter (Potamogeton crispusL.) processing were examined using six outdoor experimental streams. Downstream portions of two streams were dosed at c. 10 μg l−1 Total Residual Chlorine (TRC), one stream at 64 μgl1, and one stream at 230μg l−1. Two control streams were not dosed; upstream riffles of each stream served as instream controls.2. Two 35 day litter breakdown (per cent AFDW remaining) experiments indicated significantly lower decay rates in the high dose riffle. No other concentration of chlorine significantly affected decay rate.3. A third experiment, conducted in medium and high dose streams, indicated that high dose chlorine exposure reduced litter decomposition rates significantly, and reduced microbial colonization, microbial electron transport system activity, and microbial litter decomposition after 4 days but not after 11 days of exposure. The number of amphipod shredders colonizing litter bags was also reduced significantly with high chlorine dose.4. A fourth experiment, after dosing was terminated, provided direct evidence that amphipod shredders were important in facilitating litter decomposition: litter bags stocked with amphipods had significantly higher decomposition rates than bags which excluded shredders.5. Overall results indicate that the high dose (c. 230 μgl−1 TRC) of chlorine reduced litter processing rates partly by reducing initial microbial conditioning, but primarily by reducing the colonization of amphipod shredders.

BEDLOAD TRANSPORT IN A POOL‐RIFFLE SEQUENCE OF A COASTAL ALASKA STREAM1

ABSTRACT: A Helley‐Smith pressure differential bedload sampler was used to measure bedload transport at consecutive riffle sections of a riffle‐pool‐riffle sequence on Bambi Creek, a small (154 ha), second‐order stream on Chichagof Island, Alaska, during four storms over a 2‐year period. Maximum bedload transport rate measured was 4920 kg/h at a streamflow of 2.35 m3/s corresponding to a storm having a 5‐year return interval. Transport of larger sediment (&gt; 8 mm) varied systematically with streamflow at the two sampling locations. At flows up to approximately bankfull, transport of large sediment was greatest at the upstream site; at flows above bankfull, transport of large sediment was greatest at the downstream site. The net import of large sediment to the pool during moderate stormflows and net export of large sediment from the pool during flows above bankfull may be related to a “convergence” or “reversal” of competence between the upstream riffle and subsequent pool at flows approximating bankfull stage. Cross‐sections monitored within the study reach indicate that stormflows resulted in net filling of the riffle sections and net scour of the pool; periods of low streamflow resulted in net scour of the riffles and net filling of the pooL

Changes in the Downstream Insect and Amphipod Fauna Caused by an Impoundment with a Hypolimnion Drain1

Mill Creek, a small stream in southwestern Wisconsin, was impounded to create a 150-acre recreational lake. This action had a pronounced effect on the insect and amphipod fauna of a riffle 600 ft below the dam, and also affected the fauna of a riffle 2 miles downstream. Prior to impoundment both riffles were inhabited by a diverse insect fauna that included mostly Ephemeoptera, Trichoptera, Diptera, and Coleoptera. After impoundment the reduction of the number of species in the upstream riffle was highly significant, and previously dominant families such as Baetidae (Ephemeroptera), Hydropsychidae (Trichoptera), and Elmidae (Coleoptera) were replaced mostly by increased populations of Simulium vittatum Zetterstedt (Diptera), Gammarus pseudolimnaeus Bousfield (Amphipoda), and Chironomus and other genera of Chironomidae (Diptera). The impoundment affected the downstream riffle similarly, but to a lesser degree. Large increases in phosphorus and nitrogen concentrations and increased siltation as a result of impoundment were probably the most important factors in altering the insect and amphipod populations.