Utah Lake, part 1: What is its natural condition?

 

Utah Lake as seen from Mt. Timpanogos. (Dan Carter/Flickr)

Part 1 of a series on Utah LakeThe following observations and insights concerning Utah Lake have been developed during my nearly 50 years in Utah Valley as a professor, researcher, environmental engineer, consultant, and local citizen.


Utah Lake is a major physical feature and an unusually complex lake ecosystem covering about half of Utah Valley’s floor area. When “full,” it averages about 9 feet deep and covers about 150 square miles. Attitudes towards Utah Lake range from “priceless, beautiful lake” to “worthless, swampy pond.” Intense competition to benefit from the lake is complicated by its rich ecology that contests many proposed ideas. The use of Utah Lake resources faces complexity and generates controversy due to the lake’s eco-richness, importance, and value. Overall, public consensus favors persistent pollution-control efforts and continuing reasonable steps to protect the lake and its tributaries. Recently, however, one feels increased public desire for wise use of public funds and to avoid expensive pollution-control programs that exhibit diminishing—even negative—returns.

Natural Condition

Utah Lake is a shallow, basin-bottom lake in a semi-arid climate. Sediments thousands of feet thick lie beneath much of the lake. By nature, the lake is turbid, slightly saline, and biologically very productive (eutrophic). In most aspects, its hydrology and ecosystem have changed little since it “stabilized” when Lake Bonneville last receded some 10,000 years ago. It is sobering to ponder that over hundreds of thousands of years, this huge lake has cyclically filled Utah’s Great Basin, often accompanied by glaciers creeping down from the mountaintops—with dry valley land and habitable conditions, similar to those found today, occasionally occurring between inhospitable climatic periods.

The Utah Lake sub-basin was formed by large geologic block movements (earthquakes). The last major faulting episode some 8,000 years ago deepened the main lake as much as 20 feet in some areas. If this had not occurred, the lake would have largely filled with sediments by now and its remnant would be just swampy borders to an extension of the Jordan River.

Since pioneer settlement about 165 years ago, indications are that Utah Lake water quality has changed little. However, land use changes, water diversions, and introduced plants and animals have caused significant changes in ecosystems in and around the lake, as well as along most tributaries.

Depth Fluctuations

The natural lake’s outflow rate was once controlled by its water level relative to a rock sill that crosses beneath the Jordan River about 7 miles downstream from the lake at Indian Ford Park. Dredging of the Jordan River channel in the late 1980s altered this condition. Nowadays, when the outlet gates are fully open, discharge down the Jordan River is determined by the lake level relative to the elevation of irrigation diversions about a mile downstream from Indian Ford.

Even over a few months, it is essentially impossible to keep the lake within a couple of feet of a desired depth, since the amount of water required is too large to quickly bring in or move out of the lake, particularly during the spring and summer months. Evaporation of about 4 feet per year also contributes to the fluctuation. Note that the 4 feet of evaporation is about 300,000 acre-feet, about one-third of lake-full volume and about one-half of the average annual inflow.

Under natural conditions, in a given year the lake level commonly varied 2 to 4 feet due to the cycle of spring runoff followed by smaller inflows and large evaporation during summer and fall. Soon after pioneer settlement, efforts began to control outflows and operate the lake as a reservoir for irrigation and other beneficial uses. Adjacent land owners strongly contested this practice that occasionally flooded thousands of acres of fertile land. These conflicts led to the establishment of “Compromise” lake elevation—the water elevation where outlet gates legally must be fully open to allow unimpeded outflow. Outlet works and other upstream storage and diversions typically add annual lake level fluctuations of 1 or 2 feet, increasing current total annual lake level fluctuations to a range of 3 to 6 feet.

During major droughts, evaporation often exceeds inflow and the lake shrinks, even when there is no surface outflow. At its lowest point the lake is very small, becoming a large pond only 3 or 4 feet deep. Wet and dry cycles vary in magnitude and length; recently cycles bottom about every 20 to 30 years. The main inflows during extreme droughts are mineral springs under and around the lake; these issue from deep sources that are less affected by droughts.

Turbidity and Odor

Utah Lake is naturally turbid, primarily due to dissolved salts precipitation and re-suspension of flocculent bottom sediments by frequent, rather high, waves on this shallow lake. The result is a milky gray-brown to a milky gray-brown-green color much of the time.

The natural precipitates are mainly marls—consisting largely of calcium carbonate mingled with lesser amounts of other minerals, particularly silica and phosphorus. Although larger tributaries contribute eroded upland soils, these contributions are minor compared to the precipitated sediments. Aquatic plant and animal detritus constitutes an even smaller amount.

Salt balances identify that about 65% of inflowing calcium and 55% of inflowing carbonate precipitate in the lake. On average this is about 92,000 tons of precipitates per year. Spread over the total lake area, this is about 2 inches every 100 years when moderately compacted. The Jordan River carries away the remaining dissolved salts. Deeper areas of the lake fill with sediments more rapidly than do shallow areas. This is due both to more precipitates from the larger volume of overlying water and the migration of sediments from near-shore shallows where wave agitation re-suspends them almost continuously. Over time re-suspended sediment tends to migrate to deeper waters. This results in filling rates in the main lake of some 3 inches per 100 years.

As to odors, Utah Lake is eutrophic, meaning its biological growth and biomass are high. Growth and decay of plant and animal life sometimes result in “swampy” conditions. Considering its very high biological productivity, these conditions are relatively infrequent in the lake due to its well-mixed, well-aerated nature. But at times and in some locations aesthetics suffer, anoxic zones develop, and bad odors are generated. This natural problem is marginally intensified by human-caused pollution.

Algae

Algae are a natural and vital part (base) of aquatic food chains, similar to grass in aquatic ecosystems. Sometimes excessive algal growth occurs; these algal “blooms” sometimes cause water quality and habitat quality problems analogous to problems caused by tangles of plants and weeds on land. Dissolved oxygen is sometimes depleted algal blooms decompose. Occasionally, released toxins are also a problem during large-bloom decay.

Utah Lake naturally grows abundant algae. The growth is not usually extreme since the lake’s natural turbidity limits sunlight penetration, thus moderating algal growth. Some algae found in the lake, particularly during the late summer and early fall, are cyanobacteria (blue-greens) that can become dominant and be particularly troublesome, even poisonous, at times. Toxins from decaying blue-green algae degrade naturally and usually dissipate within a few days.

A good indicator of past water quality conditions in a lake is the types and numbers of tiny diatom algae (their shells) deposited over the years in the layered bottom sediments. Studies of sediment cores from Utah Lake indicate that types and relative amounts of diatom algae have not changed significantly over the last few thousand years. Since algae types and relative numbers are rather sensitive to changes in water quality and other aquatic conditions, the relative uniformity found in the lake sediments is strong evidence that environmental and water quality factors in Utah Lake have been rather constant, even until now, for at least a few thousand years.

Salinity and TDS

From a water quality and beneficial use viewpoint, Utah Lake’s TDS levels are slightly high. During an average year, about half of the lake’s inflow evaporates and thus nearly doubles its total dissolved solids (TDS) concentrations. TDS has ranged from 500 to 2000 mg/l and averaged about 900 mg/L over the last 83 years. (For comparison, seawater TDS is about 35,000 mg/l). Nearly 30% of inflowing TDS is precipitated (mostly calcium and carbonate) and settles to the bottom.

A major TDS source is numerous small mineral springs that flow into the lake. These mildly thermal, slightly salty springs—with a TDS range of 1,500 to 7,000 mg/L—are associated with geological faults around and under the lake. These mineral springs contribute only 5% of the inflowing water but 27% of the salts. Most of these mineral springs are small, scattered, often diffuse and submerged. These factors make it infeasible to undertake large-scale collection and export of these salty waters to reduce salt loading to the lake.

Modeling simulations of the lake indicate that average TDS concentrations are now some 35%–45% higher than pre-settlement times. The increase is mainly due to (1) diversions of tributary waters that reduce the inflow of low-salt waters and also reduce lake flushing and (2) increases in TDS concentrations in tributary waters due to upstream water use. The salt increase does not seem to have caused significant problems to the lake’s ecosystem.

merritt_graph

Results from the LKSIM model that simulates water balance and salt concentrations in Utah Lake.

Photo license: CC BY-NC-SA 2.0

LaVere B. Merritt

LaVere B. Merritt is a professor emeritus of civil and environmental engineering at Brigham Young University. His research and public service have included many multidisciplinary studies on Utah Lake. He served as member and then chair of the Provo Metropolitan Water Board for many years and is a consultant to both public and private entities on Utah Lake matters.

Comments are closed