8274

• UCREFRP Catalog Number: 8274
• Author: Pitlick, J. and R. Cress.
• Title: Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics.
• USFW Year: 2000.
• USFW - Doc Type: Recovery Implementation Program Project 48-C,
• Copyright Material: NO

A second, system-wide trend revealed by these data is the distinct period from the late 1950s
through the late 1970s when annual sediment loads were much lower than the period before or
after. Pitlick et al. (1999) have previously suggested that significant amounts of sediment would
have been deposited in the channel during this time, which may have had long-lasting effects on
native fish populations. The statistical significance of these differences was evaluated here using
the Mann-Whitney test, a nonparametric procedure that is appropriate for situations in which the
assumptions of standard parametric procedures are not satisfied (e.g. if the data are not normally
distributed). The Mann-Whitney test is essentially a test of differences in means (similar to the t-
test), but rather than comparing the data themselves, the test examines differences in the ranks of
individual values (Berenson et al., 1988). Two samples are considered to be statistically different if
they contain disproportionate numbers of highly or lowly ranked values. Comparisons were made
between two periods, 1934-58 and 1959-97; the State Line gauge was excluded from the analysis
because the record includes only five years prior to 1958.
a) Colorado River at Glenwood Springs
a
1.5
c
m
1.0
c
m
CL
0.5
w
150
E 0.0
m
? 100
m
r
a 50
a
c
c p
1930 1940 1950 1960 1970 1980 1990 2000
b) Colorado River near Cameo
A
3
4.0 m
3.0
0
m
CL
2.0
a
A 30G 10
5
m 0.0
200
L
V
0 100
W
7
C
C Q
a
1930 1940 1950 1960 1970 1980 1990 2000
Figure 14. Trends in annual discharge and suspended sediment load at (a) Glenwood Springs and
(b) Cameo. Horizontal lines indicate means for the periods 1934-1958 and 1959-1997.
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