Geology and Hydrology of Salt Lake County 101

 

Genevieve Atwood, October 2007

Chief Education Officer, Earth Science Education

Adjunct, University of Utah, Department of Geography

To view this talk on the web, go to http://www.earthscienceeducation.org/ and follow the links.

 

The purpose of this talk is to give some perspective, some rationale of why Salt Lake County’s watersheds are the way they are, specifically why Great Salt Lake is located where it is; why the Jordan River flows north; why some places are better than others for landfills; and how climate has affected the region.

 

Here are three take-away messages about Salt Lake County’s geology and hydrology:

 

·       Geology 101, Earth materials: The bedrock of Salt Lake County and its watersheds is diverse and includes every major bedrock type (sedimentary, metamorphic, and igneous) and diverse sediments deposited by all five agents of erosion/deposition: wind, water, glacial ice, mass-wasting, and humans. 

 

·       Geology 101, Landscapes: Tectonics sets the scene. Erosion and deposition modify the scene. Extensional tectonics of the past 20 million years has caused Salt Lake Valley to drop. Climate change largely determines rates of erosion. Climate change of the past 35,000 has altered regional hydrology dramatically with cold, wet, Ice Age Lake Bonneville in contrast to hot, dry, Holocene Great Salt Lake.

 

·       Hydrology 101: Ground water and surface what of Salt Lake County are intimately connected and are determined by geologic setting (materials and landscape) and by climate. The hydrology of Salt Lake County is not static.

 

By the end of this talk, you should understand this schematic of Salt Lake County’s ground and surface water regimes. LINK ArnowThiros schematic

 

FIRST – location, location, location.

LINK to USGS Tech Pub 31 map of major drainages in Utah.

LINK to USGS Tech Pub 31 map of Jordan Valley (Salt Lake Valley) major surface water drainages

LINK to PRWUA - JVWCD Map of watersheds of Salt Lake County

 

Salt Lake County is located across the boundary of two of North America’s contrasting regions: Utah ’s 3 physiographic provinces

What this means to Utah = 3 sets of resources; 3 sets of scenery; 3 sets of geologic hazards.

With respect to watersheds:

·       Rocky Mountain region = rivers run from it;

·       Colorado Plateau = rivers run through it;

·       Basin and Range = rivers run to it.

 

http://130.166.124.2/utah_panorama_atlas/page20/files/page20-1021-full.html used with permission William Bowen, 2006. SLCo-page20-1021

 

Rocky Mountain physiographic province – Provo River watershed

Rocky Mountain region = high terrain with massive complexes of mountains and ranges above 9,000 ft a.s.l.; humid continental – hot summer climate, more precipitation than evaporation; snow in winter; recharge from snow melt; vegetated valleys and parks; extensive watershed catchment areas; major recharge areas for Salt Lake Valley aquifers; major sources of ground and surface water for Salt Lake County communities; water quality – naturally good quality although affected by geology, issues of mining, non-point sources, and point sources; evidence of both erosion and deposition processes; issues of climate change.

Basin and Range physiographic province

Bounded by Wasatch fault on the east; broad basins and narrow, north-south trending mountain ranges; arid to semi-arid climate; climate zone = steppe (less precipitation than evaporation); limited precipitation; ephemeral drainages from ranges, winter snow with some recharge; thunderstorms cause rapid runoff and debris flows; limited as ground water sources for Salt Lake County (Magna, Herriman?, Kennecott properties?); water quality issues affected by geology, residence time, mining; erosion in ranges but region dominated by deposition in broad; closed basins, issues of climate change.

LINK – UGS(Stantec) geologic units of SLCounty – affects water quality and water flow

LINK – USGS – Bedrock versus basin fill – affects groun water quality (somewhat) and water flow (hugely) LINK ESE schematic cross-section

 

SECOND – Tectonics and landforms

Why are there contrasting physiographic provinces? Different geologic histories.

Plate tectonics LINK

Far western U.S. (western California, etc) moving west faster than Mid-West and East.

 

LINK to schematic of extension of Basin and Range

Rocky Mountain physiographic province tectonic setting… pretty quiet; rides high like a massive iceberg with a huge “root” (isostatic equilibrium).

Basin and Range physiographic province = extensional tectonics.

 

 

THIRD – Climate change

Given that tectonics results in closed-basins, we get lakes. Closed basin lakes are historians of climate change. As colder, wetter, cloudier climate drives the lake upward (1) wave processes create shoreline expressions, and (2) rivers carry sediments and dissolved constituents into the lake and they are deposited as layers across the lake bed. Note: these layers are important to Salt Lake County hydrology. Clays of the lake bottom become “aquitards” meaning water passes through them slowly, in contrast to “aquifers” that yield water quickly and are sources of ground water.

 

LINK Great Salt Lake – Lake Bonneville extent

LINK to USGS Tech Pub 31SLCounty precipitation

 

FOURTH – ground and surface water hydrology of SLCounty.

Analysis of the Arnow, 1983 (USGS Tech Pub 31) – Thiros, 2003 (USGS WRI-034325) schematics

LINK to schematic in color (Thiros, 2003)

LINK to schematic in black and white (Arnow, 1983)

LINK to water table

LINK to aquifers-aquitards

LINK to confined versus unconfined aquifers

LINK to potentiometric surface

LINK to plan view of groundwater flows

LINK to colored version

 

LINK to summary of ground water and surface water relationships