100
North Fork
South Fork
Bay Fringe high
Outlet x-section area (m2)
SF
y = 0.80x0.96
R² = 0.82
NF
y = 0.52x0.98
R² = 0.69
BF
y = 0.09x1.03
R² = 0.58
Bay Fringe low
10
Stilly high
Stilly low
Stilly high
y = 0.78x0.65
R² = 0.66
1
Stilly low
y = 0.12x1.21
R² = 0.76
0.1
0.1
1
10
100
y = 0.28x1.20
R² = 0.77
Outlet x-section area (m2)
0.88-1.56 m
NAVD88
y = 0.60x1.24
R² = 0.82
1.66-2.22 m
NAVD88
10
100
2.26-2.92 m
NAVD88
y = 0.07x1.16
R² = 0.76
1
0.1
0.1
1
10
100
Outlet width (m)
Figure 1. Scaling of outlet cross-section area with tidal channel outlet width (an indicator of
channel size) for the Stillaguamish and Skagit deltas (top frame) and for all channels binned by
elevation range (bottom frame). The modifiers “high” and “low” refer to relative elevation.
Note that regression slopes ( = power function exponents) are similar within the Skagit Delta, but
vary widely in the Stillaguamish Delta (data collected for a separate, but related project).
However, when the data are binned by elevation (measured on the marsh surface adjacent to the
channel cross-section) the regression slopes are relatively uniform (1.20 ± 0.04). The results
suggest that as sea level rises (or as relative marsh elevation becomes lower) tidal channel crosssection area, and thus habitat volume for rearing juvenile Chinook and other estuarine species,
will decline. For the purposes of the current project however, these relationships will provide
reference conditions against which to measure recovery of dike breach and removal sites.
Outlet cross-sectinon area (m2)
100
North Fork
South Fork
Bay Fringe high
Bay Fringe low
Stilly high
Stilly low
10
NF
y = 1.69x0.44
R² = 0.64
SF
y = 3.02x0.48
R² = 0.63
BF
y = 0.70x0.21
R² = 0.14
Stilly high
y = 1.64x0.35
R² = 0.49
1
Stilly low
y = 0.18x1.12
R² = 0.69
0.1
0.01
Outlet cross-section area (m2)
100
0.1
1
10
0.88-1.56 m
NAVD88
10
y = 1.56x0.37
R² = 0.23
y = 2.48x0.47
R² = 0.61
1.66-2.22 m
NAVD88
100
2.26-2.92 m
NAVD88
1
y = 0.58x0.40
R² = 0.26
0.1
0.01
0.1
1
10
100
Drainage basin area (ha)
Figure 2. Scaling of outlet cross-section area with estimated drainage basin area for the
Stillaguamish and Skagit deltas (top frame), and for all channels binned by elevation range
(bottom frame). The modifiers “high” and “low” refer to relative elevation. The patterns are
similar to those in the previous figure, but the independent variable is estimated drainage basin
area. Drainage basin area was estimated from drainage divides calculated in GIS under the
assumption that they were equidistant between adjacent channel networks (vide Rinaldo A, S
Fagherazzi, S Lanzoni, M Marani. 1999. Tidal networks 2. Watershed delineation and comparative
network morphology. Water Resources Research 35:3905–3917). Compared to figure 1, the separation
between high, medium, and low marshes is not as clean and R2 values are lower, probably because of the
derived nature of the independent variable and because drainage basins (especially large ones) encompass
a range of elevations.