This paper synthesizes the literature on step pools, with particular reference to theory construction and explanatory models. The paper proceeds from a discussion of the fundamental aspects of step pools to a clarification of terminology.
The influence of pool length on local turbulence production and energy slope : a flume experiment
To investigate the influence of pool elongation on turbulence production and energy slope, a flume experiment was conducted with a pool geometry that was loosely based on a 0.10 scale model of a forced pool from North Saint Vrain Creek, Colorado
. Based on the overall flow pattern, the strongest turbulence occurs in the center of the pool along the shear zone between the jet and recirculating eddy. The lateral location of this shear zone is sensitive to changes in pool length. The results indicate
some form of hydraulic optimization is possible with pools adjusting their length to adjust the location and strength of turbulent intensities in the center of pools, and lower their rate of energy dissipation.
Morphological effects of local scouring in step-pool streams
In this paper, natural step-pool geometry is compared with steep alluvial channels where grade-control structures such as check-dams and bed sills make the stream profile resemble a natural stepped stream. In order to test the hypothesis
that natural pools are analogous to pools formed below grade-control works with respect to their dimensions, shape and formative dynamics, 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern Italian Alps. The results
lend support to an upstream-forced cascade model for step-pool formation, where the energy of falling jets controls the geometry of the pools, and is therefore regarded as the most important scaling-independent variable.
Alluvial architecture in headwater streams with special emphasis on step-pool topography
This study sought to develop a better understanding of channel types in mountainous watersheds with an emphasis placed on step-pool reaches. Specifically the AA. sought to determine the stream gradients associated with various types of self-formed
channel architecture (pool-riffle, plane bed, step-pool and cascades) and to develop relationships which could be used in predicting step-pool geometry and in understanding formative mechanisms.
Sediment routing hypothesis for pool-riffle maintenance
Bank erosion ; California ; Channel geometry ; Hydrodynamics ; Model ; Sediment load ; Sediment transport ; Spatial variation ; Step-pool channel ; Stream ; Tracer ; United States of America
This paper provides evidence that sediment routing around pools is a key mechanism for pool-riffle maintenance in sinuous upland gravel-bed streams. 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. 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
Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels
To better understand pool geometry and sorting characteristics of New England channels, characterizations of pool geometry were performed on 145 different pools strongly influenced by channel constrictions. The results suggest that pool dimensions
are related to basin-wide variability and local hydraulic conditions. The findings also suggest that the characteristics of constrictions and the downstream section of pools provide an important control on scouring and sorting characteristics throughout
Geometric adjustment of pools to changes in slope and discharge : a flume experiment
To characterize the factors controlling pool shape, 30 different forced pools were created utilizing a 50% triangular constriction in a 0.5-m wide, 6-m long recirculating flume. Pools were scoured from an initial plane bed of sand with a d of 0.25
mm. Pool depth and length were measured and used as dependent variables in least-squares, multiple regression analyses. Discharge, channel-bed gradient and energy slope were the independent variables. Additional linear-regression analyses were
conducted with either pool depth or length and stream power. Results indicate that both pool depth and length are primarily a function of discharge.
Pool and riffle characteristics in relation to channel gradient
The channel gradients, along three rivers in coastal northern California, discussed in this paper are higher than those often reported for channels with pool-riffle, rather sequences. However, the AA. have designated these as pool-riffle, than step
-pool, channels, because of the presence of a strong lateral flow component, and the lack of well-organized bed-steps spanning the entire channel width.
The morphologic structure of step-pools in mountain streams
The purpose of this paper is to report a field-based investigation on the geometric structure of step-pools in the Santa Monica Mountain streams. The step-pool morphology is evaluated empirically in light of variations in a downstream direction
. To that end, a conceptual model of process-form linkages is proposed, whereby the step-pool morphology is produced by positive relationships between wavelength and discharge, and between height and particle size.
Young dry grassland ecosystems in Denmark : development in soil nutrient pools and root characteristics
This paper explores the soil nutrient pools of 14 young dry grasslands and compares them to the nutrient pools on heathlands, forested and agricultural land. On 2 sites, the plant species composition is compared to characteristic species for the EU
Habitats Directive in Denmark and the relationship between soil nutrient pools and underground biomass development is investigated. Root density was high in this study and soil nutrient content appeared to be an important parameter determining root
Random controls on semi-rhythmic spacing of pools and riffles in construction-dominated rivers
A simulation model for pool and riffle formation is used to demonstrate that semi-rhythmic spacing of pools with an approximate spacing of 5 to 7 bankfull widths can be recreated from a random distribution of obstructions and minimum pool
- and riffle-length criteria. It is assumed that a pool-riffle couplet will achieve a minimum length based on dominant-discharge conditions. Values for the minimum-length assumption are based on field data collected in New England and California, while
the theoretical basis relies on the demonstrated hydraulic response of individual pools to elongation. Discussion of the results.
Velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions
A model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit-slopes of pools. According to the model, a narrow zone of high velocity occurs in the center
of pools, creating scour. The model is tested with field and flume measurements of velocity, water-surface elevation, and size of bed sediments in recirculating-eddy influenced pools. Local reversals of the water-surface gradient were measured in the field
Towards a theory for step pools in stream channels
This paper synthesizes the recent literature on step pools, with particular reference to theory construction and progress made over the past decade and a half. Four groups of contribution are identified. The first focuses on understanding
morphological relations in order to seek causal mechanisms. The second emphasizes processes that result from the step-pool configuration. The third is a group of integrative studies that seek broader explanations for step pools sequences. The fourth and growing
body of literature applies what has been learned about step pools toward managing and restoring steep channels.
Flow structure and development of circular meander pools
In this study, flow structure, patterns of erosion and deposition, and bank strength in a circular meander pool are examined in order to understand better the conditions that lead to their development. Several possible modes of development
An appraisal of the velocity-reversal hypothesis for stable pool-riffle sequences in the River Severn, England
The literature in support of the hypothesis together with published adverse criticism is reviewed. These observations are substantiated by a detailed study of the hydraulic geometry of stable pool-riffle sequences in the River Severn, close
Secondary flows and the pool-riffle unit : a case study of the processes of meander development
Major bed forms of bed-gravel rivers may be resolved into pool-riffle units. Their identification and observation demonstrate links between bedform, flow pattern and channel change. Each unit seems to be associated to a systematic pattern
Differential bed sedimentology and the maintenance of riffle-pool sequences
The purpose of this paper is to clarify several aspects of the sedimentology of riffle-pool sequences, and to suggest how these both reflect and control riffle-pool stability in the presence of spatial differences in the turbulent near-bed flow