9476

• UCREFRP Catalog Number: 9476
• Author: Colorado Foundation for Water Education.
• Title: Citizen's Guide to Colorado Water Law.
• USFW Year: 2003.
• USFW - Doc Type: \
• Copyright Material: NO

lnterstate and Federal Law
The Imperial Dam at Yuma: Arizona (top)
diverts Colorado River water to the All-American
Canal to irrigate fields in the Imperial Valley in
Culi fornia. Interstate agreements govern how
water is shared among the states. The system
of pumps and canals that make up the Central
Arizona Project (CAP) is monitored (rorn a
control roor~n in Phoenix (above). The CAP car-
ries water frs~rr~t the Colorado River:
lnterstate Compacts and Treaties
Colorado River Compact, 1922
La Plata River Compact, 1922
South Platte River Compact, 1923
Rio Grande River Compact, 1938
lnterstate Compacts,
Equitable Apportionment
Decrees, and Treaties
Colorado must live within its water
constraints. The first and most basic con-
straint on water use within the state is the
amount of rainfall and snowfall that occurs
each year.
The second constraint is legal:
Colorado's obligation to deliver water to
downstream states under interstate water
compacts and United States Supreme
Court equitable apportionment decrees.
International treaties with Mexico also
affect Colorado's water use.
The unbridled ability of states to allo-
cate and govern water use within their
states halted early in the 20th century. In
1907, the Supreme
Republican River Compact, 1942
Arkansas River Compact, 1948
Upper Colorado River Compact, 1948
Amended Castilla Compact, 1963
Animas-LaPlata Project Compact, 1968
U.S. -Mexican Water Treaty, 1906
Rio Grande, Colorado and Tijuana Treaty; 1944
Court in Kansas v.
Colorado held that all
states sharing a stream
system were entitled to
an equitable share of
river water.
Under equitable
apportionment, the
U.S. Supreme Court
has authority to allocate a state's share of
river water from time to time based on
another state's need, if a state files directly
with the high court.
The compact clause of the United
States Constitution allows states to fix
their allocations in perpetuity by con-
tract, with Congressional approval. An
interstate compact is both state law and a
law of the United States. This promotes
long-term planning and reliability, and
diminishes the rush to develop water as
soon as possible.
Due in great measure to the efforts of
Dolph Carpenter, son of a Weld County
homesteader, Colorado entered into nine
interstate water compacts as alternatives to
court-apportionment.
Because of interstate and international
requirements, and because Colorado has
not yet fully developed all of its water allo-
cations, Colorado passes a large amount of
water out of the state. In an average year,
approximately 16 million acre-feet (maf)
of water flows in Colorado rivers. After
accounting for all the water consumed for
beneficial use, the state's rivers deliver an
annual average 9.248 maf to the Pacific
side of the Continental Divide, anal 1.478
maf to the Atlantic side. Up to two-thirds
of Colorado's surface water is obligated to
downstream states and Mexico.
Drought can greatly alter the amount
of water Colorado produces for in-state
and out-of-state use. For example, in the
drought year 2002, natural flows in
Colorado rivers were closer to 4 maf, com-
pared to the 16 maf average.
Approximately 6 maf of reservoir storage
was used up in 2002 to supplement these
meager natural streamflows.
For a summary of the compacts, equi-
table apportionment decrees, and treaties,
see www.cwcb.state.co.us/Fact_Sheets/
Compact_Facts.pdf.
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