Stop 1: The Gallatin River

for Air Photo
Stop 1 topographic portion Modified from Topo II
To better orient yourself, take a look at the trip log page!
contour interval 40 feet

The History of the Gallatin River:

Gallatin River, looking southeast toward Big Sky

The history of the Gallatin Valley is intrinsically tied to the Gallatin River. Cut and fill events have taken place throughout the Quaternary history of the valley. These cut and fill events, we will find, are very important in terms of water availability and rights. Before we can begin to discuss such events, we must first understand the nature of the Gallatin River.

Tasks!
Find yourself on the topographic map. Using the geologic map , determine which rock unit is the Gallatin River flowing over?	Alternative geologic map?
Why are we doing this task? Field geologists begin their observations of an outcrop or landscape with several pieces of very important information: 1. determine your location on a topographic map; with this information we can begin to understand the landscape, the gradient of rivers, elevation change in an area, etc. 2. determine what rock unit you are standing on or near; with this information we can determine the age and depositional environment of a certain rock unit. 3. sketch the topography and add rock units; with this information we can begin to understand the geologic history of an area

Formation of alluvial aquifers:

By definition, alluvial aquifers are deposited by rivers. Although the Gallatin river may not be actively depositing an alluvial aquifer today, it is important to understand how it was deposited. Once we understand the role of the Gallatin River in the formation of the West Gallatin Alluvium, we can use it as a modern analog for the formation of older gravel aquifers. This process of comparing present depositional environments to past environments is called the principle of uniformitarianism.

Tasks!
Look at the photo of the braided river in Alaska (this river is actively depositing). image courtesy of University of Cincinnati Compared to the braided river which is actively depositing sediment, does the the Gallatin River seem to be actively depositing sediment ? What is your evidence?	To learn more about alluvial aquifers

Aquifer formation and groundwater transport:

Now that we have an understanding of how alluvial aquifers are formed, we can begin to use uniformitarianism to understand how older aquifers are created. We can also use our modern analog to begin to understand how groundwater might move through aquifers formed in a similar fashion.

Gallatin River, looking southeast toward Big Sky

Tasks!
Geologists often make landscape sketches as well as taking written notes. By sketching an outcrop or landscape, they are forced to make careful observations of scale, texture, grain size, grain shape and more.
Draw the river (the Gallatin River). Make a note of the location of specific grain sizes and what controls the location of grain size across the river.	What kind of sketch do you mean?
Imagine an older alluvial aquifer deposited in such a manner. Would the ground water move better through the gravel and sands or the silts? Look at the images below for help!
How will the distribution of grain size effect groundwater transport in the future?

image courtesy of USGS Water Science for Schools

Move on to discover the vast difference between alluvial (unconsolidated) aquifers and crystalline rock.