An Introduction to the Physical Basis for Deriving River Information by Optical Remote Sensing - Fluvial Remote Sensing for Science and Management - page 58

0.4

2.5 2.5

Wet gravel
Periphyton
Wet limestone
Woody debris
Senescent Veg.
Dry Cobble
Dry Sand

Wet gravel
Periphyton
Wet limestone
H 2 O absorption

0.35

2 2

0.3

0.25

1.5 1.5

0.2

1 1

0.15

0.1

0.5 0.5

0.05

0 0 0

400 450 500 550 600 650 700 750 800 400 450 500 550 600 650 700 750 800

Wavelength (nm) Wavelength (nm)

(a) (b)

Figure 3.8 (a) Reflectance spectra for various in-stream and terrestrial features observed along a gravel-bed river. (b) Spectral
differences in bottom reflectance are minor relative to spectral differences in attenuation by the water column, which is dominated by
absorption by pure water. Data on the absorption coefficient for pure water a w (

) are from Pope and Fry (1997) and Smith and
Baker (1981). Reproduced from Legleiter, C.J. et al. (2009) Spectrally based remote sensing of river bathymetry. In: Earth Surface
Processes and Landforms , pp. 1039-1059, with permission from John Wiley & Sons, Inc.

λ

algorithms tend to yield under-estimates where the river
flows over bright limestone and over-estimates in areas
of darker gravel; more sophisticated spectrally-based
methods are more robust to these effects (e.g., Lee
et al., 2001; Mobley et al., 2005). Although few field
observations of bottom reflectance in natural river
channels have been made, the most extensive data set of
which we are aware indicated that bottom reflectance was
actually quite homogeneous in a fairly typical gravel-bed
river (Legleiter et al., 2009). One hundred and thirty-nine
ground-based reflectance measurements are summarised
in Figure 3.9, which strongly resembles the periphyton
spectrum in Figure 3.8a, implying that algal coating of
the streamed was pervasive during late summer. This
result is fortuitous for estimating depth because variation
in bottom reflectance apparently need not be a concern,
at least in this environment. Streambed photographs
acquired along with the field spectra featured diverse
lithologies and a range of particle sizes, but these
grain-to-grain differences were not significant at the scale
of the reflectance measurements (0.21m field of view).
The most salient feature of these spectra was a strong
absorption band (i.e., decrease in reflectance) centered
at 675 nm due to absorption by algal chlorophyll. Strong
spectral signals of this kind will be most evident at
shallow depths, where water column exerts less of an
influence, and more subdued in deeper water.

0.08

75%
50%
25%

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0

400 450 500 550 600 650 700 750 800 850 900

Wavelength (nm)

Figure 3.9 Field spectra measured along a gravel-bed river by
Legleiter et al. (2009). The thick white line represents the
median of 139 spectra and the solid areas encompass the
indicated percentage of the distribution of reflectance
measurements. These spectra represent a range of depths but
have a similar spectral shape, indicating that bottom
reflectance is fairly uniform and dominated by periphyton.
Reproduced from Legleiter, C.J. et al. (2009) Spectrally based
remote sensing of river bathymetry. In: Earth Surface Processes
and Landforms , pp. 1039-1059, with permission from John
Wiley & Sons, Inc.