FLOOD01653

• County: Larimer
• Community: Larimer County
• Stream Name: Big Thompson River
• Title: Evaluation of the Flood Hydrology in the Colorado Front Range Using Streamflow Records and Paleoflood Data for the Big Thompson River Basin
• Date: 5/1/1986
• Prepared For: USGS
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

In this type of paleoflood investigation, lack of evidence of the

occurrence of extraordinary floods is as important as discovering tangible

onsite evidence of such floods. This is true because the geomorphic

evidence of extraordinary floods in steep mountain basins, such as the

upper Big Thompson River, is unequivocal, easy to recognize and long-

lasting because of the volume and size of sediments deposited (Jarrett and

Costa, 1984). Knowledge of the nonoccurrence of floods for long periods of

time (in this instance, since post-glacial time) has great value in extend-

ing flood-frequency relationships to great recurrence intervals (Stedinger

and Cohn, 1986) and provides a physical basis for the nonoccurrence of

exceptional floods for very long periods of time.

In the upper Big Thompson River basin, the strategy was to visit the

most likely places where evidence of large floods might be preserved, had

they occurred. The experience gained from investigating landforms and

deposits of the 1976 Big Thompson flood (Costa, 1978b) and the Lawn Lake
Dam failure in the upper Big Thompson River basin (Jarrett and Costa,
in press) was used to guide the investigations. Sites studied include:

(1) Locations of rapid energy dissipation, where coarse sediment would be

deposited, such as tributary junctions or abrupt large valley expansions;

(2) locations downstream from moraines across valley floors where large

floods would be likely to deposit sediments eroded from the moraines; and

(3) locations along the sides of valleys in wide, expanding reaches where
sediment would likely be deposited.

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