SPATIO-TEMPORAL DYNAMICS OF EDGE HABITAT IN NATURAL AND
ALTERED SPRING SNOWMELT RECESSION FLOW REGIMES, SIERRA
NEVADA, CALIFORNIA
CAROLINE WALLIS
Institute of Science and the Environment, University of Worcester, Henwick Grove,
Worcester, WR2 6AJ, United Kingdom
IAN MADDOCK
Institute of Science and the Environment, University of Worcester, Henwick Grove,
Worcester, WR2 6AJ, United Kingdom
SARAH YARNELL
Centre for Watershed Science, University of California, 1 Shields Avenue,
Davis, 95616, California, United States
Edge habitat, defined as the relatively shallow, slow zones in channel margins, provides important flow
refugia and nursery habitat for juvenile fish, amphibians and riparian species [1,2,3,4]. The
Mediterranean-Montane rivers of the Sierra Nevada foothills, California, have a spring snowmelt flow
regime. The timing and rate of change of the recession limb has been shown to shape local ecological
processes, such as reproductive cues for the threatened foothill yellow-legged frog (Rana boylii)
[4,5,6,7]. It was hypothesised that flow regulation would alter the availability, location and spatial
dynamics of edge habitat during the recession limb. A hydraulic patch dynamics framework, outlined
in [8], was applied to a regulated (Middle Fork) and unregulated reach (North Fork) of the American
River, California to evaluate potential differences in edge habitat dynamics. Repeat surveys of the
location and hydraulic characteristics (depth, mean column streamwise and lateral velocity and velocity
variability), of the edge habitat zone, delineated in the field by relative differences in depth, velocity
and/or surface flow type conditions from the main channel, were conducted throughout the spring
snowmelt recession limb (April–August 2011) using a 2D Acoustic Doppler Velocimeter. This paper
presents the results of the patch dynamics analysis, highlighting differences in the number of hydraulic
patches and their associated hydraulic characteristics as well as the change in location and extent of
edge habitat throughout the recession limb. The ecological implications of flow regime alterations on
edge habitat dynamics and suggestions for how these could be minimised will also be discussed.
1.
INTRODUCTION
The adverse impacts of dams and river regulation have been widely documented and continue to be
researched [9,10,11]. In the 1990s the importance of natural flow variability in maintaining the
ecological integrity of rivers gained widespread acceptance and has become a guiding paradigm for
river restoration and environmentally sensitive flow management [12,13]. Scientists and water resource
managers, however, must continue to work closely to ensure appropriate quantitative evidence of the
physical and ecological effects of altered flow regimes is collected and incorporated into flow
management plans [14,15,16].
Recent research into the ecology of the spring snowmelt recession flow regime, characteristic
of rivers in the Californian Mediterranean-Montane climate, has linked the timing, magnitude and rate
of change of flows to the success of native species [6,7]. The timing of peak snowmelt flows provides
the reproductive cue for cottonwood (Populus spp.) and the threatened foothill yellow-legged frog
(Rana boylii) [17,18]. Peak flows scour riparian zones, resetting succession and carrying an influx of
nutrients which aid the growth of new seedlings [19]. Foothill yellow-legged frogs only breed after the
risk of potentially scouring peak flows has passed. Females then lay their eggs on substrate or
vegetation in well oxygenated but relatively sheltered water near the channel margin. Crucial to the
success of both species, whose life cycles have adapted to the flow regime in this region, is the
availability, persistence and predictable shift in location of edge habitat during the subsequent flow
recession. Edge habitat, variously defined as the relatively shallow, slow waters near the channel
margins [1,2,20], provides ideal spawning and nursery habitat. If flows recede too quickly after peak
spring runoff, egg masses and seeds are prone to dessication and tadpoles/juvenile fish to stranding.
The ecological importance of edge habitat is recognised however relatively little research has
investigated how flow regulation effects the hydraulic characteristics, availability, and spatio-temporal
dynamics of edge habitat during the spring snowmelt recession. A limited number of hydrodynamic
modelling studies suggest that flow regulation can severely reduces edge habitat persistence and disturb
the seasonal pattern of edge habitat availability [1,2]. Further research could provide important
information to dam operators seeking to minimise ecological impact whilst balancing multiple water
resource needs.
This research was part of a wider project investigating the relationship between hydraulics,
ecology and geomorphology during the spring snowmelt recession at six sites across two catchments in
the Sierra Nevada foothills, California. The specific aims of research presented in this paper were to (a)
characterise and delineate hydraulic patches in the edge habitat zone of an unimpaired and impaired
reach, and (b) to explore the effects of flow regulation on the location, availability and spatio-temporal
dynamics of edge habitat and its constituent patches. It was hypothesised that whilst the total extent of
edge habitat might be relatively similar at each site, the hydraulic characteristics and change in location
of hydraulic patches within the edge habitat zone would vary between sites, reflecting the differences in
bar morphology and flow regime.
2.
METHODS
2.1
Study area
The research was conducted in the American River watershed which drains a 5,568km2 area of the
north western slopes of the Sierra Nevada mountains in northern California. The American river is
heavily modified in two of its three main forks; flows are regulated to produce hydropower and provide
suitable conditions for white water rafting during summer. The Mediterranean-montane rivers in this
region are characterised by a spring snowmelt recession hydrograph. Peak flows typically occur
between March-May when air temperatures are sufficient to melt the high elevation snowpack.
Discharge recedes throughout the summer drought months, reaching baseflow in August. During this
study, owing to the unusually cold, wet spring of 2011, peak snowmelt occurred in mid to late June.
The falling limb of the recession was steeper and of shorter duration than average and the tail of the
recession limb extended to early September 2011.
Study sites were selected in the North and Middle Forks. The North Fork (NF_AMR) flows
through a steep, narrow, wooded valley and is unimpaired. Mean daily flow during 2011 water year
was 43.6m3s-1. The Middle Fork (MF_AMR) is heavily regulated by a series of dams licensed by the
Federal Energy Regulatory Committee. In 2011 near-natural patterns of flow variability were
maintained until mid July, after which a hydropeaking regime was operated resulting in diurnal
fluctuations of 24 m3s-1 (10-34m3s-1). Mean daily flow during 2011 water year was 50.6m3s-1.
The head of a lateral bar was selected as the focal point of each study reach. Lateral bars
experience considerable wetting and drying as flows vary and so provide an ideal location for studying
the dynamics of edge habitat. The MF_AMR reach extended 120m and contained a pool tail out with a
large recirculation zone upstream of the lateral bar (left bank) and a large vegetated mid-channel bar
which split the channel adjacent to the head of the lateral bar. The bar was dominated by cobble-gravel
substrate and covered with a layer of silt at the head. It varied in width from 50-75m. The profile of the
bar shelved gently at its head but a sharp drop-off was evident in the cross-channel profile where the
bar was adjacent to the split channel. Dense, established willow vegetation was present throughout the
reach. The NF_AMR reach, scaled to the channel width, extended 96m, and incorporated a pool tail out
at the head the lateral bar, a run and a rapid. The lateral bar extended the full length of the reach on the
right bank, shelving gently from a bedrock outcrop at the channel margins into the main channel. The
bar was 15-25m wide, composed of coarse cobble substrate and sparsely vegetated with willow
saplings. Inter-site differences were inevitable but minimised where possible.
2.2
Data collection
Six surveys of the spatial extent and hydraulic characteristics of the edge habitat zone were carried out
in each reach between April-August 2011 at a range of discharges representative of (a) peak snowmelt,
(b) high flow during the declining limb of the recession, (c) moderate flow at mid-recession and (d) the
hydropeaking regime in the impaired reach and the tail of the recession limb in the unimpaired reach.
Edge habitat was delineated in the field based on relative differences in depth, velocity and/or surface
flow type from the main channel. Water depth and mean column velocity (streamwise and crossstream) at 0.6depth, were measured at 0.8m/1m cross-stream and 4m/5m streamwise intervals at the
NF_AMR and MF_AMR sites respectively using a Sontek FlowTracker 2D Acoustic Doppler
Velocimeter.
2.3
Data analysis
A hydraulic patch dynamics framework presented in [8] was applied to evaluate differences in edge
habitat dynamics. Hydraulic data were combined by site and standardised (z-scores) to account for
different scales of measurement. Each combined dataset was clustered using the fuzzy c-means [21]
and Gath-Geva [22] algorithms in the Fuzzy Clustering and Data Analysis Toolbox for MATLAB [23]
to delineate relatively homogeneous hydraulic patches and the transitional zones between them. Seven
partitions of each combined dataset were derived, each with a unique number of clusters (c) in the
range 2≤c≤8. The fuzziness value (m) was held constant at m=2. Fuzzy clusters were defuzzified to
define crisp hydraulic patches using a 0.7 α-cut threshold and Confusion Index (CI>0.6) rule [24,25].
Hydraulic patches and transition zones were mapped at each flow using ArcGIS v.10 (ESRI,
2010) and exported to FRAGSTATS v.3.3 [26]. Thirteen spatial metrics quantifying patch diversity,
patch geometry and patch configuration were calculated within the edge habitat zone at each discharge.
Permutation-based resampling techniques were used to test for statistical differences in hydraulic patch
diversity in the edge habitat zone between discharges and sites. Patch configuration was explored using
non-metric multidimensional scaling (MDS) and multivariate statistics in PRIMER v.6.1 [27]. GIS
overlays were also used to calculate the percentage change in location of the boundary and constituent
hydraulic patches of the edge habitat zone with every increase in discharge [28].
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