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size data set, Davey and Lapointe (2007) adapted and
extended the original concept to account for sedimentary
links of lower mountain landscapes of North Eastern
Canada where coarse sediment inputs are often related
to supply zones (rather than point sources or nodes)
originating in bedrock canyon reaches or valley bottom
deposits of glacial drift.
Because the downstream changes in substrate and asso-
ciated slope along sedimentary links are accompanied by
changes in channel morphology and hydraulics, they
create a longitudinal sequence of aquatic habitat types
moving from steep, fast flowing and turbulent boulder
bed channels at the head of links to meandering, slow-
flowing, low-gradient sand channels at the downstream
end. Rice et al. (2001) were the first to demonstrate
the usefulness of sedimentary links for ecological mod-
elling by applying the concept to explain the longitudinal
structuration of benthic macroinvertebrate communi-
ties. Davey and Lapointe (2007) then showed how such
information on the large-scale variations of substrate
size could help understand the spatial organisation of
Atlantic salmon spawning habitat. Studying the location
of Atlantic salmon (
Salmo salar
) spawning sites along
eight sedimentary links of the Sainte-Marguerite River,
they found that the centroıd of the spawning sites on each
link tended to occur towards the middle to downstream
end of the cobble-gravel fining sequence (Figure 9.8b).
Within these zones, D50 was always in the suitable range
of 40-60mm. Higher upstream, bed material size was too
coarse to allow female fish to dig their redds. Below, the
absence of spawning activity was probably related to poor
embryo survival associated with the high percentage of
sand in riffle substrates.
Similar progress in the understanding of the large-
scale pattern of juvenile salmon spatial distribution was
obtained by analysing the spatial correspondence between
semi-continuous measures of parr density and grain
size variations of the sedimentary links of the Sainte-
Marguerite River (Figure 9.8a). Clearly, peaks in parr
density correspond to the heads of sedimentary links
where boulder-rich reaches offer both good summer
feeding habitats and abundant bed interstices providing
winter shelters to juveniles.
These two examples demonstrate that the grain size
information derived fromhigh resolution images of rivers
can help quickly identify 'hot spots' in the production of
salmonid within the riverscape, even for rivers where
only few or sparse fish or habitat information is currently
available.
25
20
15
10
5
0
D50 [cm]
Figure 9.7
Example of a merged wet/dry grain size map
obtained from the airborne method. The central portion of the
channel corresponds to a dry exposed gravel bar. Lighter blue
and yellow/orange colours correspond to coarser bed material.
on the Ste-Marguerite River. Using historical fry and parr
density data obtained from 1997 to 2004 at 48 parcels
(5m
×
20m) distributed along the river, they derived
substrate preference models using substrate size (D50)
measurements obtained, 1) directly inside the parcel at
time of density estimation using the traditional Wolman
count method and 2) inside the larger grain size map
of the image including the fishing parcel obtained using
the automated airborne grain size mapping methods.
They showed that, although the shape of the relationships
between juvenile salmon density and D50 were similar
for the two models, the relationship was stronger using
mean image D50, suggesting that the habitat surrounding
the location of the fishing parcel had a direct effect on
fish density. Clearly, this example shows that one benefit
of automated methods of grain size measurements is to
allow multi-scale analysis of fish habitat relationships, a
possibility that would be labour intensive using traditional
ground based methods.
The grain size profile information obtained from the
automated grain sizingmethods allowed the identification
of distinct sequences of downstream grain size fining
along the Sainte-MargueriteRiver. Indeed, it was observed
that, rather than exhibiting a single longitudinal decrease
of grain size from headwater to mouth, the river could be
segmented into a number of discrete sedimentary links,
each characterised by a node of coarse sediment supply
followed by a gradual downstream fining of substrate.
The sedimentary link concept was originally developed
for high mountain river environments where the supply
of coarse sediment is mainly related to tributary inputs,
valley-side landslides and tributary fan contacts (Rice
and Church, 1998). However, using the Geosalar grain
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