What Does A Crest Mean In Hydrology?

Colorado River crested during Hurricane Harvey flooding the surrounding river banks.
Colorado River crested during Hurricane Harvey flooding the surrounding river banks.

Some terms used in hydrology can be very confusing and hard to understand especially if one is not an expert in the field. When talking about the rising waters of river banks, common words in hydrology include "CFS", "datum points", and "river crest”. Understanding what the terms CFS and datum points refer to is important to explaining the meaning of a crest.

Defining These Terms

A river crest refers to the highest point the water level is expected to rise. In other words, it is the maximum height of a river or a lake. The term is usually limited to floods.

CFS is the short form of cubic feet per second. It is used to measure the flow of the river or the amount of water increase in a lake. The CFS of a river may decrease or increase depending on several factors such as the amount of rainfall or melting glacier, the number of tributaries and how much water is withdrawn from the river.

The datum point refers to the height of the river and the depth of the lake. The datum point varies depending on the width of the river. The datum point is loosely understood to mean the normal height of the river.

Relationship Between CFS, Crest, and Datum Point

Hydrologists and disaster management services use the three features to determine the likelihood of flood and its impact. If the crest of the river is far beyond the datum point then the river is likely to burst its bank. The crest is measured from the datum point and not from the riverbed or lake bed. For example, when a geologist says the crest is 6ft then he means the river has risen 6 ft above the normal flow. During floods, the CFS increases and the crest rises. If a river discharges into a lake at 10,000 CFS and another river drains the lake at 5,000 CFS, then 5,000 CFS is left in the lake and the crest of the lake is expected to rise above the datum point.


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