7832

• UCREFRP Catalog Number: 7832
• Author: U.S. Bureau of Reclamation.
• Title: A Pilot For Long-Term Monitoring Of Resources On The Colorado River In The Grand Canyon Using Geographic Information Systems.
• USFW Year: 1995.
• USFW - Doc Type: \
• Copyright Material: NO

Hypsometric Analysis. -A preliminary analysis of recirculation zone hypsometry was conducted
to quantify sediment storage changes within the recirculation zones. Hypsometry is the
distribution of area with respect to elevation within a defined region. Hypsometric curves have
traditionally been applied to drainage basins (Strahler, 1952; Schumm, 1956; Bloom, 1991). In
this application, we define the hypsometries of individual recirculation zones along the Colorado
River. By using dimensionless parameters, hypsometric curves from individual eddies can be
compared irrespective of true scale. We develop a general model of sandbar hypsometry and
compare the hypsometries of sandbars throughout this system during the period of interim flow.
TIN (triangulated irregular network) models of the combined topographic and bathymetric data
set were constructed within a boundary delineating the recirculation zone. Arc coverages of
0.5-m contour lines are generated from the TIN models and the area enclosed by each contour
line is summed to generate the raw hypsometric data (fig. 11a). The raw hypsometric data are
then normalized by expressing individual values as a ratio of the total range, and the cumulative
relative height versus the relative area is plotted (fig. 11b).
A system-wide hypsometric curve was generated by pooling all terrestrial and bathymetric data
from 30 sandbars for six survey runs from July 1991 to April 1994 (fig. 12). This curve was best
described using a third order polynomial equation:
Ai = 0.991 + (0.146 * E) - (2.535 * E;2) +(1.401 * Eis) (1)
(r2 = 0.895)
with A, as the relative sandbar area in hypsometric interval I, and Ei as the relative elevation
of interval I. The elevation intervals used in this analysis were 0.05 m.
An index of the geomorphic condition of individual sandbars was developed by comparing the
individual site hypsometry to the system-wide mean. Residuals between the observed
hypsometric curve and the system-wide hypsometric curve were calculated at each elevation
interval for each survey. These residual values were totaled to generate the SHCR (summed
hypsometric curve residual) for each survey. We then analyzed SHCR as a response variable
in a three-factor analysis of covariance (ANCOVA, SYSTAT v. 5.1). This analysis included the
following predictor variables: sampling run, reach width (wide versus narrow), and depositional
environment (separation, reattachment, or eddy bar), with distance from Lees Ferry and the
lagged, or antecedent, SHCR value as covariates.
The ANCOVA results show that antecedent bar condition (p < 0.0001) and reach width (p =
0.025) were the significant variables influencing sandbar morphology through time. The strong
correlation between SHCR and antecedent SHCR is best described using a least squares simple
linear regression:
SHCRn _ (0.849 * SHCRn_1) - 0.209 (2)
(adjusted r2 = 0.813, F, 61= 201.8,p < 0.0001)
The lower values along this line indicate sites near the system-wide mean sandbar morphology,
and the length of the line represents the range of bar morphologies in the system during the
period of study.
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