Saturday, January 23, 2010

Storms and flooding across Arizona

The following summary and assessment of this week's storms was prepared by Dr. Phil Pearthree, Chief of the AZGS Environmental Section:

The combination of several days of some antecedent moisture and then prolonged, occasionally intense rainfall starting January 22 generated large floods on many of the gaged drainages in central Arizona. Generally, the areas that received abundant moisture stretched from the lower Colorado River Valley over to Maricopa County, but the largest amounts of precipitation occurred in the central mountains across the state. The higher country above the Mogollon Rim received lots of snow and there were concerns about the possibility of rain-on-snow flooding in the large Verde River tributaries like Oak Creek, but evidently it stayed sufficiently cold that this did not happen in any significant way.

AZGS compiled a summary table of peak discharges from Thursday (1-21-10)  and Friday morning using USGS preliminary gage data (with a little FCDMC data), and compared these peaks to the historical maximum peaks for each drainage. The streams or gage locations and more detailed information can be found at Kyle House has pointed out on his blog ( that you can also bring these stations into Google Earth.   No new records were established, although a couple of streams came very close to record discharges (i.e., upper Agua Fria, Big Sandy). In the Verde River system, the large (100k+ cfs) peak at Tangle Creek was almost entirely derived from the watershed below Verde Valley, demonstrated by the later and much smaller peak at the Verde River gage below Camp Verde (at Chasm Creek) and the paltry floods on Oak Creek and its "siblings". This phenomenon is unusual in the historical record, as most large Verde River floods have been produced primarily by the tributaries draining the Mogollon Rim in Verde Valley, but the largest peak discharge in January 1993 had a similar genesis as the flood this morning. Note that with floods this large, we expect USGS hydrologists will visit the gage sites and make slope-area-based indirect discharge estimates that may change the "real" peak discharge values.

Generally, because of storage capacity in dams on the major rivers and flood control work over the past few decades in Phoenix, flood damage along the major rivers in Phoenix will be limited or nonexistent. There may be problems along some of the more natural stream reaches (Wenden along Centennial Wash in west-central Arizona is an example), but generally flood damage should be limited. There are probably countless rockfalls or mud deposits along highways traversing the central mountains, such as I-17, SR 87, and US 60. Continued wet weather could result in landslide activation as well.

With the saturated conditions in central Arizona, more storms would obviously have the potential to generate floods. As of now the medium-term forecast is fairly benign, but we will be keeping an eye on that.

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