TWENTIETH CENTURY ARROYO CHANGES IN CHACO CULTURE NATIONAL HISTORICAL PARK
 
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By Allen C. Gellis



ABSTRACT
 
Chaco Wash arroyo channel changes in the 20th 
century have become a major concern of the National 
Park Service. Several archeologic and cultural sites are 
located in the Chaco Wash corridor; thus, increased 
erosional activity of Chaco Wash, such as channel 
incision and increased meandering, may affect these 
sites.

Through field surveys, photogrammetric 
analyses, and reviews of existing reports and maps, 
arroyo changes at Chaco Culture National Historic 
Park were documented. Arroyo changes were 
documented for the inner active channel and the entire 
arroyo cross section. The inner channel of Chaco Wash 
evolved from a wide, braided channel in the 1930's to 
a narrower channel with a well-developed flood plain 
by the 1970's. From 1934 to 1973 the active channel 
narrowed an average of 26 meters, and from the 1970's 
to 2000 the channel narrowed an average of 9 meters. 
Overall from 1934 to 2000, the inner channel narrowed 
an average of 30 meters.

From 1934 to 2000, the top of Chaco Wash 
widened at four cross sections, narrowed at one, and 
remained the same at another. The top of Chaco Wash 
widened at a rate of 0.4 meter per year from the 1970's 
to 2000 compared with 0.2 meter per year from 1934 to 
1973. At 50-percent depth or halfway down the arroyo 
channel, four cross sections widened and two cross 
sections narrowed from 1934 to 2000. Rates of 
widening at 50-percent depth decreased from 0.2 meter 
per year from 1934 to 1973 to 0.1 meter per year from 
the 1970's to 2000. From 1934 to 2000, arroyo depth 
decreased at five of six cross sections and increased at 
one cross section. Arroyo depth between 1934 and 
1973 decreased an average 1.4 meters from 
aggradation and between the 1970's and 2000 
increased an average 0.4 meter from channel scour.

From 1934 to 2000, arroyo cross-sectional area 
decreased at all six cross sections. Cross-sectional 
areas in Chaco Wash decreased from 1934 to 1973 as a 
result of sediment deposition and both decreased and 
increased from the 1970's to 2000. The cross-sectional 
area decreased by the 1970's due to channel narrowing 
and flood-plain formation. Increases in cross-sectional 
area are from channel scour and channel widening. 
Photogrammetric analyses of volumetric changes for a 
1.7-kilometer reach of Chaco Wash showed sediment 
deposition from 1934 to 1973 of 64 square meters per 
unit length of channel over 1.7 kilometers to erosion 
from 1973 to 2000 of 7 square meters per unit length of 
channel.

Chaco Wash evolved from a braided channel in 
the 1930's to a narrow, sinuous inner channel by the 
1970's. Chaco Wash was widening in the 1930's, 
leading to sediment deposition and formation of an 
inner flood plain. Channel narrowing resulted from 
increased sediment deposition on the flood plain. 
Sediment deposition may be related to a decrease in 
peak flows, an increase in flood-plain vegetation, or an 
increase in the transport of fine-grained sediment. 
Increases in bankfull depth of Chaco Wash between the 
1970's and 2000 were due to aggradation of the flood 
plain and channel scour. Thus, rates of aggradation and 
cross-sectional filling were greater from 1934 to the 
1970's than from the 1970's to 2000.



Abstract from Water-Resources Investigations Report 01-4251