by Christine Albano
Urban storm drain runoff events are a major contributor of nutrients as well as physical change to streams like Emigration Creek. The urban drainage area going into Emigration Creek is heavily developed. This means that there are a lot of materials going into the creek that might not be present under natural conditions. In addition, urban areas increase the amount of runoff going into streams because much of the area is covered by asphalt and concrete which makes the ground impervious to water absorption. This causes an increased amount of physical disturbance to the stream through scouring, erosion, and deposition during large storm events. This has many effects on the aquatic ecosystem of the stream.
The total drainage area for Emigration Creek is 18.0 sq miles (1). Out of this, 1.5 square miles of this area is urban drainage that goes into the storm drains while the remainder makes up the canyon watershed. The storm drains in Salt Lake City are designed to have the capacity to handle runoff from the amount of water expected during a 100 year, 24 hour storm event (2). For the Emigration Creek drainage area, the expected amount of rain in this event is 3.5 inches. Much of this water is absorbed into the ground but the rest will flow into storm drains. For the 1.5 square mile urban drainage area east of 1300 East, there are 25 pipes of various sizes that drain into the creek with a combined capacity of 20,036.24 cubic feet (click here for table of pipe data) (reference 3).
In Salt Lake City, storm water runs directly into streams without being treated. In some other cities storm water and sewage water are mixed together and treated before being released. The method used here in SLC is more cost efficient but it is dependent upon preventative measures for ensuring water quality in streams. The EPA and the Salt Lake Dept of Public Utilities regularly monitor the storm water outfalls for things such as Biological Oxygen Demand, Oil and Grease, pH, coliform bacteria, and total suspended solids.
Revision of the Clean Water Act in 1991 requires states to conform to storm water regulations such as the NPDES (National Pollutant Discharge Elimination System). This regulates many of the preventative measures taken to protect stream waters. In urban areas, every construction site must apply for a permit and show compliance with storm water regulations. This prevents large amounts of suspended solids from going into streams. Although these solids arent toxic they are considered pollutants because of how they affect the ecology of the stream.
Anything that goes into the gutter from yards, driveways, and streets ends up in storm drains. This can include oil and antifreeze from cars, fertilizers, pesticides, and other hazardous household wastes that are not supposed to go into the drains, but inevitably do. In addition, soil and organic nutrients, pet feces, sediments and other dissolved solids are washed in during storm events. These all affect the water chemistry of the stream. Sometimes nutrients can be beneficial to the productivity of the stream but when they are added in excess, or toxic chemicals are present, they can have effects on the aquatic ecosystem and the stream can become polluted. This type of pollution is called non-point source pollution. Non-point source pollution is the result of runoff of excess nutrients and pollutants that does not come from one particular place, but from many places. According to the State of Utah Division of Water Quality, non-point source pollution has had the biggest impacts on Utahs water quality (4).
Due to the use of Emigration Creek as an urban storm water collection system, and urbanization of the upper canyon watershed, the stream is prone to repeated physical disturbances during storm events. This is especially true in the fall and spring when the most precipitation and runoff events occur. Even during minor rain events, pulses of runoff cause erosion and deposition in the stream. An example of scouring from storm runoff can be seen on the Westminster Campus where a major storm event during the 1983-84 (100 year flood) caused so much runoff that the culvert beneath 1300 E did not have the capacity to allow the water to flow. The water backed up and was forced through with so much pressure that it scoured a deep pool, forever changing that part of the streambed at the west end of the 1300 E. culvert.
As discussed earlier, runoff also carries suspended solids which either remain suspended or are deposited when the velocity of the stream decreases. In both cases there are effects on the aquatic ecosystem.
Storm drain runoff affects the physical and chemical characteristics of the stream which in turn affect the organisms that live there. In small amounts,these effects are not always negative but because Emigration Creek is in an urban area it is constantly being altered further away from its natural state by human activities.
When suspended solids are carried in by storm water they cloud the water which prevents light from getting to the algae living there. This prevents algae such as diatoms and Cladophora from photosynthesizing and the productivity of food for the stream is halted or slowed. Even when these solids settle, they blanket on top of algae growing in the stream, which has the same effect. This affects the organisms higher in the food web that use algae as a food source such as Mayflies and Caddisflies and other aquatic insects.
When storm events occur many organisms that don't have adaptations to hold onto a substrate are carried away downstream. This can include segmented worms, snails, fish, and any other organism that can't hang on. In addition, anything suspended in the water such as bacteria and fungi are also washed downstream. In the fall, much of the productivity of the ecosystem is dependent on dead leaves and other organic detritus coming from the riparian trees. Storm events wash this food source away which effects detritovores such as the leech and Dugesia.
Excess organic and inorganic (such as nitrogen and phosphorous ) nutrients can pollute the stream because they can't be used by the organisms in the stream as fast as they are put into the stream. When there is an abundance of nutrients the algae is able to flourish and it literally takes over. This is called eutrophication and it prevents other organisms from surviving in the stream. Fortunately, Emigration Creek does not have this problem. In the case of Emigration Creek, addition of nutrients from urban storm runoff is probobly beneficial to the productivity of the ecosystem because nutrients aren't being added in excess.
Because the water from Emigration Creek is not used for drinking, toxic chemicals going into the water don't endanger humans but do put other organisms in danger. One of the problems with toxic chemicals going into the stream is that they can poison the fish that live there such as the trout.
1) Call, Charles H. Pd.D., Student Civil Engineering Univ. of Utah. Research Proposal. "Watershed model of the interconnections between quantity and quality of runoff from Red Butte and Emigration Canyons." http://www.cc.utah.edu/~chc02760/p_tab_2.html
2) Salt Lake City Corporation, Dept. of Public Utilities Engineering Division, Drainage Regulation No. SW-1. Sept. 1996.
3) Salt Lake City Corporation, Dept. of Public Utilities Engineering Division, Storm Drain Location Maps. updated Dec. 6, 1996
4) Utah Dept of Environmental Quality, Division of Water Quality. "Utah's Water Quality Fact Sheet." http://www.eq.state.ut.us/eqwq/96fact_w.htm
Special Thanks to Nick Kreiger of SLC Dept of Public Utilities, GIS Section, who provided the Storm Drainage Basin Map. August 19, 1997.