GLG 455/598 Advanced Field Geology

March 7, 8, 1998

Along the Agua River at this site, an excellent fluvial terrace sequence and evidence for the high water levels in past floods are preserved. Mapping will include documentation of the distribution of the preserved fluvial landforms and consideration of the soil development and potential ages of the terrace deposits. Furthermore, we will document the high water marks in the channel, survey their locations and channel reach geometry (slope and cross-sections) using the Total Stations. These data will be used in application of simple channel hydraulics (Manning equation and slope-area method) to estimate the magnitude of the discharge in the past floods. Those results can be compared with existing gauge data and more sophisticated paleoflood calculations.

Location: West of I-17 (Table Mesa exit) along a straight reach of the Agua Fria River at the Boulder Creek Ranch (part of Lake Pleasant Regional Park).

Mapping goals: Map the distribution of the different Quaternary geologic units, establish their relative ages, consider the implications for landscape development, and determine the recent flood history of the Agua River in the mapping area.

Mapping and notation strategy; Document the following:

1) Distribution of rock types. Where are the main units, what are they (describe them carefully and completely), and what did they look like when they were deposited or formed? How were they deposited or formed? What are the clast types (if relevant)? What are the sedimentary structures (if relevant)? Concentrate on the fluvial terraces and their relationships to the underlying rocks.

2) Soil descriptions. Use the experience from the Biscuit Flat project to consider the relative ages of fluvial terraces by comparing their soil development.

3) Surface pavement development and surface roughness. What is the size distribution of clasts on the surfaces and to what degree are they varnished? How does the pavement and the meter-scale surface roughness develop over time? What processes operate over what surfaces?

4) Vegetation. Try to estimate the density and type distribution of the main vegetation types on the surfaces.

5) Landform development. What and where are the major landforms? You should map the major channels and slope breaks in the area.

6) Major landforms. Map major landforms and features in the area that may be useful to define the context.

7) Quality control and coverage. Be sure to quality control your observations. Try not to leave any gaps on the map. Make lots of careful observations reported clearly in your notebooks.

Questions to address in the write-up (Due March 31 by 5 PM to Arrowsmith)

(should be five pages plus any figures [which will include your field maps, any other sketches of key relationships that illustrate your points, including calculations and figures for the flood discharge calculations]).

1) What is the geologic history of the mapping area? What are the relative ages of the units and the landforms? How do the Quaternary gravels, etc. relate to the older basalts and other rocks?

2) How do fluvial terraces and the associated deposits relate to the underlying rocks? How have the fluvial terraces developed? Do you have any ideas to address their formation processes? As the river cuts down, how do the nearby hillslopes respond?

3) Based upon the survey data that we generate, what are the magnitudes discharges of the largest historic floods along the Agua Fria? What are the magnitudes discharges of the largest PRE-historic floods along the Agua Fria? What is the evidence for the floods? Be sure to spend some time discussing how your observations relate to those flood events and try to describe how the channel looked at those different flood stages. Try to use the slope-area method to calculate the discharge. What can you say about the fluvial sediment transport during those events relative to the sediment that you observe on the channel floor.

4) If we can provide you extra information about flood discharge along the Agua Fria, compare those results with your calculations and observations.