Geography Reference
In-Depth Information
are available from the United States Department of
Agriculture (USDA) Farm Service Agency (FSA) Aerial
Photography Field Office (APFO) at www.apfo.usda.gov.
Many historical images within the United States and
Canada are archived at the state and provincial level,
often at the map libraries of major public universities.
These are often stored as hard-copy images and must be
scanned and georeferenced before being useful for digital
analysis.
Satellite imagery is available at a global level starting in
1972 with the launch of the LandSat mission. Individual
LandSat scenes as well as mosaics from the mid 1970s,
are available at the EROS Earth Explorer site. Ground
resolution ranges from 57 m for earlier Multispectral
Scanner (MSS) images to 14.25 m for Pan Sharpened
ETM
polygons having a biophysical meaning (water channel,
islands, gravel/sand bars, active or abandoned channels)
and the analysis of their geometric properties (area, length
of edges, width, etc.) (see review of Gilvear and Bryant,
2003). Images are often not the only source of infor-
mation and usually complement data provided from
vectorial layers or from ground measures. Both are easily
integrated using a GIS. Such external data (e.g. which
are not provided by images) can also be introduced to
validate or predict information from characteristics of
observed features.
Planform geometry of meandering rivers has been cov-
ered by many authors since the 1970s, both at the bend
and the reach scales (amplitude/wavelength of meander,
radius of curvature, sinuosity, etc.) (Lewin, 1977, 1987,
Hooke, 1984; Hooke and Redmond, 1989). Multivariate
characterisations of meandering rivers have been explored
by Howard and Hemberger (1991). Multi-threaded chan-
nels are also well studied (e.g., braided index) (Hong
and Davies, 1979; Egozi and Ashmore, 2008). Common
indices characterise the extent of braiding and/or anas-
tamosing, the relative amount of edges, islands, gravel
bars, etc. In specific contexts (e.g., very high resolution
imagery, transparent flows free of surface distortion),
pools and riffles, or, more generally, in-stream habitat
can also be delineated (Figure 11.1).
When several image series are available, it is possible
to observe changes in channel planimetry or channel
pattern on annual to decadal timescales (Odgaard, 1987;
Piegay et al., 1997; Gurnell, 1997; Gilvear et al., 2000;
Winterbottom, 2000; Burge and Lapointe, 2005; Zanoni
et al., 2008; etc.). It is also possible to predict discharge
from water flow area, notably when water surface width
is particularly sensitive to discharge change, which is the
case of wandering or braided rivers.
Several methods have been developed to quantify lateral
adjustments, based on linear rate of lateral shifting, rate of
reworked areas, or elongation rate of channel centerlines
(Aalto et al., 2008). Maps are used for getting a longer
time period but aerial photo series are usually the main
source of information when available. From the example
of the Rillito Creek, Arizona, Graf (1984) provided an
original contribution to the quantification of planform
changes through the mapping of an erosion probability
as a function of the lateral and upstream distance of a
given location to the nearest active channel assuming
continuation in the mobility trend using maps from 1871
to 1978. His method has been applied by Wasklewicz et al.
(2004) on the lower Mississippi with a slight modification,
assigning to each map a proportional weight according
images. Most of the other satellite sources, notably
the ones providing sub-metric resolution such as Ikonos,
Quickbird or Worldview-1 are commercial and do not
provide easy on line data access, and the archives, even if
they exist, are often not extensive enough to cover entire
regions (although a notable exception are SPOT satellite
mosaics for Canada available at http://www.geobase.ca).
In all cases, the horizontal precision of imagery should be
carefully evaluated before making observations of relative
geomorphic change.
Imagery can be combined with other data such as
vectorial layers corresponding to land-use types (Corine
Land-cover for example), hydrographic networks, road
layers,andDEMfromlow(25mfortheFrenchBD
topo, 90 m for the SRTM available worldwide, 15-30 m
for much of the U.S. National Elevation Dataset DEM,
etc.) to high resolution (LiDAR). A national archive of
publicly available LiDAR data in the United States is
maintained by the USGS at lidar.cr.usgs.gov, but the
archive does not necessarily provide access to datasets
stored at the state and local level. Map libraries also
often provide access to high resolution LiDAR DEM's,
which are often distributed through state or local agen-
cies. Historical maps, geological maps or discharge data
may also be available
+
from the Web (USGS NWIS;
Other
online
data
maintained
by
regional
and
local
governments).
11.3 How can we treat the information?
11.3.1 What can we see?
From archived images, it is possible to perform a charac-
terisation of the fluvial corridor based on the detection of
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