RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
3.0 BASELINE ASSESSMENT RESULTS
Watershed Conditions Bonneville Boulevard, drains cover of 9.2% (SLCO 2009). 11,189 acres of wooded Much of this impervious cover is Size and Land Use mountainous terrain ranging in associated with the paved City elevation from 9,400 to 4,300 Creek Canyon Road which City Creek is the northernmost feet. Upper City Creek flows for extends 5.75 miles up the significant mountain stream a total length of 10.3 miles canyon to Rotary Park. More drainage in Salt Lake City (SLCO 2009). The majority of than 99% of the upper (Figure 3.1). The upper the upper subwatershed is subwatershed is public land subwatershed, which includes the forested and undeveloped, with owned by Salt Lake City and RCS study area above an estimated overall impervious managed for drinking water
Figure 3.1. City Creek watershed (map from SLCO 2009). SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-1 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
source protection and open east and the State Capitol supported by groundwater space/recreation. The City Creek complex on the west. An inputs, and flows in the channel Water Treatment Plant (CCWTP) additional 2.3 miles of lower City recover rapidly below the is located about 3 miles up the Creek south and west of the RCS CCWTP diversion point. canyon; access to the area above study area is conveyed in the CCWTP is subject to engineered conduits to the Average annual precipitation restrictions designed to limit Jordan River (Figure 1.1). ranges from 32–42 inches in the potential water supply Estimated existing impervious highest portion of the upper contamination. Recreational cover is 19.7% within the lower subwatershed, from 22–32 opportunities in the upper subwatershed (SLCO 2009). inches in the mid-elevation subwatershed include biking, portions of the upper jogging, hiking, fishing, Hydrology subwatershed, and from 11 to 22 picnicking at designated picnic inches in the valley/lower sites, skiing, and snowshoeing. The hydrology of City Creek is subwatershed (SLCO 2009). altered by flow diversions at the Above the CCWTP City Creek is The lower City Creek CCWTP, which commonly classified as having perennial sub-watershed is much smaller, removes all available flows from flow; below the CCWTP the draining 4,621 acres of mixed the creek. Typical withdrawal creek is classified as use land below Bonneville rates average about 4.6 cfs, but “perennial-reduced,” indicating Boulevard. Lower City Creek rates as great as 23 cfs are that flows are artificially reduced flows approximately 1.5 miles sometimes diverted during the by the stream diversions for through Memory Grove, an area spring runoff period (F. Reynolds municipal water supply (SLCO owned and managed by the 2010, pers. comm.). In the 2009). City’s Parks Division. The vicinity of the CCWTP, the corridor is bordered by the stream is a “gaining reach” No significant water storage Avenues residential area on the where surface flows are reservoirs are currently present on City Creek, although Below: City Creek debris basin above Bonneville Boulevard shortly historically reservoirs were after dredging. constructed in the canyon to store water for municipal supply. Two debris basins located immediately above and below the Bonneville Boulevard crossing provide some degree of flow regulation and capture a significant portion of the creek’s coarse sediment loads. These debris basins are maintained by Salt Lake County and require dredging approximately every two to three years. Additionally, the formal portion of Memory Grove Park adjacent to study reach LCC_R02B is designed to function as a flow regulation
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basin that would control flood flows entering the downstream engineered conduits. This current system for flow and sediment regulation was constructed to prevent damages similar to those sustained during the major flood event in 1983.
City Creek’s hydrology is characterized by a distinct springtime peak typical of snowmelt-driven systems. Based on analysis of flow data recorded at the County gage in Memory Grove from 1980–2005, mean Figure 3.2. Monthly flows at Salt Lake County’s gage at Memory annual flow is 8.3 cfs, average Grove. monthly flow is highest in May (Figure 3.2 ), and base flows average about 3 cfs. Peak daily flow occurs on May 21st on average (SLCO 2009), and average annual high flow is 67 cfs based on analysis of the 1980–2005 time period.
Urbanization and development in the lower subwatershed have affected surface water- groundwater patterns. These influences affect the study reaches from LCC_R01B downstream, which receive water from urbanized areas via storm drain outfalls. As contributing watershed areas become converted to impervious surfaces, a greater proportion of storm water runs off as surface flow rather than infiltrating into the ground, leaving less groundwater available to supply baseflow to the creek during the summer dry Figure 3.3. Relationship between impervious cover and surface period (Figure 3.3). The runoff. Impervious cover in a watershed results relatively high proportion of in increased surface runoff. Diagram and caption text from FISRWG 1998.
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surface flow during storm events fishery). Below the CCWTP, the Jordan River, several new E.coli can also lead to a “flashy” designated beneficial uses monitoring stations have been hydrologic pattern with rapid, include 2B and 3A. City Creek’s recently established on City brief rises in flow during storms water quality is considered good Creek by DWQ and Salt Lake (Figure 3.4). On City Creek, and fully supports all of its County. Samples are being however, these influences are beneficial uses (DWQ 2006). collected monthly at these sites, limited only to the downstream- but data have not yet gone most study reaches because the Potential nonpoint sources of through quality assurance/control majority of the contributing contamination in the City Creek procedures and have not been watershed area has been watershed include fertilizers and released to the public or protected as undeveloped open herbicides from managed parks, comprehensively analyzed. As space. runoff from roads/urbanized additional data are collected and areas (e.g., automotive fluids), analyzed and as DWQ’s new E. Water Quality and waste from wildlife (e.g., coli work group becomes more deer, ducks) and pets. established, it is anticipated that a Above the CCWTP, City Creek’s Bacteriological contamination is better understanding of beneficial use classifications as of particular concern because the bacteriological contamination designated by the Utah Division Jordan River, the receiving body sources within City Creek will of Water Quality (DWQ) include for City Creek water, is currently develop. 1C (high quality drinking water), listed as impaired for E. coli 2B (secondary contact (DWQ 2006). To help inform Geology and Soils recreation), and 3A (cold water ongoing TMDL studies on the Within the upper subwatershed, City Creek flows primarily through Pennsylvanian and Mississippian age sandstones and limestones. Within the RCS study area, the channel flows through Miocene conglomerates, Oligocene and Eocene tuffs and other volcanics, and quaternary landslide deposits. Below reach UCC_R10C, these formations are surrounded by Pleistocene Lake Bonneville deposits ranging from finer-grained silt and clay deposits to coarser sand and gravel deposits. Within reach LCC_R01C and downstream, the channel flows directly through these Lake Bonneville deposits Figure 3.4. A comparison of hydrographs before and after and other quaternary alluvial urbanization. The discharge curve is higher and steeper deposits (Bryant 1990). for urban streams than for natural streams. Diagram and caption text from FISRWG 1998.
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In the upper City Creek Fish, Birds, and Wildlife surveys in this portion of the subwatershed, approximately creek (D. Wiley 2010, pers. 20–35 % of the soils have severe Above the CCWTP City Creek comm.). to very severe soil erosion supports a confirmed potential. In the lower conservation population of native During the Audubon Society’s subwatershed, 35–50% of the Bonneville cutthroat trout, the 2005 Christmas bird count, soils have severe to very severe state fish of Utah and a state- which included City Creek erosion potential (SLCO 2009). designated special status species Canyon and Memory Grove, 19 Several significant hillslope that evolved as the top predator different bird species were slumps are evident adjacent to within ancient Lake Bonneville observed (Carr 2009). Scientific the south/east side of the channel (Lentsch et al. 2000). City Creek data describing detailed within the study area. Slumps also supports rainbow trout, information about mammal have also occurred recently on brown trout, and rainbow- populations are limited. the steep hillside west of the cutthroat hybrid crosses (SLCO However, general observations channel within areas adjacent to 2009). During the RCS field suggest that wildlife populations reach LCC_R01B and assessments, trout (species in the canyon include deer, downstream. In some areas unknown) were observed bobcats, moose, mountain lions, these instabilities appear to be throughout the upper portion of coyotes, raccoons, rattlesnakes, influenced by steepened cut the study area between reaches rabbits, mice, and large numbers slopes and runoff associated with UCC_R09 to UCC_R11B. The of elk (Hooton 2000). Evidence the Bonneville Boulevard Utah Division of Wildlife of beaver was noted within roadway; other areas are part of Resources (DWR) conducted several of the upper RCS study larger natural landslide features. fisheries sampling in a 200-foot- reaches. long portion of the stream in this After Lake Bonneville receded area in 2007 and observed 19 Historical Conditions approximately 16,000 years ago, brown trout and one cutthroat and Current Trends it left a series of old shoreline trout (D. Wiley 2010, pers. deposits that now form comm.). prominent “benches” along the City Creek History edges of Salt Lake Valley. To Quantitative data on fish City Creek holds a prominent reach its modern base level at the populations within the lower place in Utah’s history, as it Jordan River, City Creek had to reaches of City Creek below became an important municipal carve through these deposits. In Bonneville Boulevard are limited. and irrigation water source part because of this natural Fish were not observed during shortly after Mormon pioneers geologic history, stream gradient RCS assessments in the lower entered Salt Lake Valley in 1847 is steep, and the channel is study reaches, suggesting that (Figure 3.5). At that time the entrenched between tall slopes population numbers may be creek was described as being that extend up to the Bonneville lower. However, fishery covered with willows and shrubs; bench levels on the east and west information was not a specific specific types of vegetation sides of the corridor within the focus of the RCS assessment and mentioned include box elder, lower subwatershed. The degree observations were merely noted cottonwood, rose briar, and of channel confinement has also when staff happened to see fish squawbush. City Creek originally been artificially increased by while collecting stream channel exhibited a braided (multi- placement of fill material for condition information. The DWR channel) plan form where it roadways. has not conducted fisheries exited the canyon. The main
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-5 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Figure 3.5. City Creek historical timeline.
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branch of the channel is Historical activities described as following a sinuous that have altered path to marshy swamp lands riparian corridor near the Jordan River (LDS conditions: CHO 1997) and flowing through a grove of cottonwoods west of • mining for iron and the Temple Block (LDS CHO other metals 1990). Pioneer accounts also remark on the abundant, cold, • logging and timber clear water in the creek. harvest Alterations to the Riparian • beaver trapping and Corridor removal Over the last 160 years, the • channel clearing and is maintained in a relatively debris removal various activities associated with development and population natural condition, and beaver populations are present in the • construction of water growth in Salt Lake Valley have supply storage reservoirs resulted in significant alterations upper subwatershed. The to the stream channel and presence of beaver is unique • flow diversion for riparian conditions of City Creek. relative to the other streams irrigation and drinking Among other factors, systematic assessed during the RCS; water programs to clear debris from however, current population channels likely contributed to levels in City Creek are most • road and stream crossing reduced riparian vegetation likely significantly smaller than construction density relative to pre-settlement during the pre-settlement era. conditions. Early citizens of Salt • residential and Lake were encouraged to gather Many of the direct alterations to commercial wood in City Creek canyon by City Creek occurred to resolve development clearing the stream of deadfall flooding problems and accommodate water supply for • introduction of invasive, (Prince and Shea 1995). Such municipal and agricultural nonnative plants “cleaning” programs involved removing debris dams and purposes. Within the lower • piping of the creek in beaver dams that increased subwatershed, the most underground conduits inundated streamside habitat significant direct changes to the area, elevated the water table, creek involved combining the • channel straightening reduced flood velocities and multiple branches of City Creek and ditch construction erosion, and trapped sediment into an artificial channel that ran and nutrients (Gardner et al. down North Temple Street • bank armoring 1999). The removal of such (Figure 3.5). By the mid-1920s structures decreased the natural this channel had been enclosed • placement of fill within “checks” on sediment and water, in underground pipes, resulting floodplain areas resulting in greater flow velocities in the loss of most of the original and streambed down-cutting streamside and riparian areas • debris basin construction (Wohl 2000). Today City Creek within the valley portion of the
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-8 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
watershed. Within both the lower and upper subwatersheds, placement of fill material for road construction has directly affected the corridor by reducing the width of the active floodplain. Channel realignments have also historically occurred in conjunction with construction of mills, water supply canals, and storage reservoirs (Figure 3.5). Following the 1983 floods, extensive channel armoring (primarily with gabion baskets) was installed to protect City Creek Canyon Road. In most cases the present-day streambed Figure 3.6. Plot of temporal trends in annual stream flow at City remains flush with the lowest tier Creek gage. Plot provided by the Salt Lake City of these gabions, suggesting that Department of Public Utilities. bed elevations have remained alterations throughout the volume, reduced springtime stable since the 1980s. watershed. precipitation amounts, and anticipated water supply The longitudinal connectivity of Recent and Anticipated shortages (Karl et al. 2009). The the City Creek riparian corridor Future Trends risk of drought, as well as the risk remains relatively intact of flooding, are expected to compared to the other streams Within both the upper and lower increase. The changes in assessed as part of the RCS. City Creek subwatersheds, land temperature will likely result in a Bonneville Boulevard is the only use predictions for 2030 do not shift in vegetation communities, major road crossing within the indicate significant changes in and altered precipitation patterns study area. However, various in- impervious cover (SLCO 2009). will influence stream hydrology channel structures have been This is a unique situation relative and channel conditions. The installed to protect utility to the other lower subwatersheds timing of snowmelt runoff is crossings including water, sewer, within Salt Lake City, where expected to occur earlier in the and petroleum pipelines. Some increases in imperviousness of spring, with an anticipated of these structures influence 12% or more are anticipated reduction in summertime base transport of sediment and debris over this time period. flows (Karl et al. 2009). It is and may impede fish passage. difficult to predict specific Channel dewatering and facilities Climate change can be changes to City Creek with associated with the CCWTP also anticipated to affect the City certainty, but recorded annual interrupt the corridor’s Creek riparian corridor. Climate stream flow volumes on City longitudinal connectivity. In projections for the southwest Creek already show a declining sum, existing channel conditions region of the United States show trend between 1970 and 2006 within the City Creek corridor increased temperatures, reduced (L. Alserda 2009, pers. comm., reflect a complex response to a mountain snowpack, a 10–20% Figure 3.6). This trend is variety of historical and ongoing decrease in annual runoff
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anticipated to continue into the flood events, dissipating velocity in riffle areas of City Creek future (Karl et al. 2009). and reducing erosion potential. (Table 3.1). Medium gravel comprises the D16 particle size at Stream and Vegetation Surveyed channel width values all cross sections, and cobble- Conditions are quite variable, ranging from sized material typically comprises about 4 to 20 feet at low flow, the D84 particle size (Table 3.1). with an average value of 12 feet Embeddedness values are highly Stream Channel (Table 3.1). During the spring variable. In flatter-gradient Characteristics runoff period, field surveys were portions of the channel such as conducted at a streamflow of 78 run and pool areas, particle sizes Salt Lake County has classified cfs, which is close to the are smaller, with fine gravel and City Creek within the RCS study 1980–2005 average annual high sand often dominant. No area as entrenched, meaning the flow value of 67 cfs. Width at consistent upstream-to- channel is vertically confined. this high flow value varies from downstream trends are evident in Composite stream stability about 12 to 35 feet, with an the pebble count results; rather, ratings for the area were good average of 19 feet. Within bed material size and during County stream LCC_R02B, channel width has embeddedness appear to be assessments completed in 2007. been directly affected by installed largely a function of local factors Also during these assessments, bank hardening measures. such as sediment inputs from the County classified City Creek erosion areas, composition of as Rosgen (1996) stream type B4 Channel slope, as determined for bank material, and the presence in reach UCC_R9, and as stream each stream reach from digital or proximity of bedrock. type B3 in the remaining RCS elevation data, varies from 2.9% study reaches (K. Collins 2009, to 4.5% within the RCS study Vegetation Characteristics pers. comm.). County bankfull area, with an average value of width estimates for the stream 3% (Figure 3.8, Table 3.1). City Table 3.2 lists all plant species reaches in the RCS study area Creek does not show any noted on the data forms during ranged from 10 feet to 23 feet, obvious consistent spatial trends the study area mapping effort. with an average value of 18 feet in gradient through the study Species are identified by their (K. Collins 2009, pers. comm.). area. Historically, channel common and scientific names, gradient would have shifted to a mapping code, wetland indicator Results of RCS field surveys and flatter, less confined, more status (USFWS 1988), and GIS analyses further illustrate the sinuous channel type in the whether the species is native to fact that the City Creek channel flatter valley area west of State Utah or introduced (NRCS is commonly entrenched and Street; however, this portion of 2009). A total of 82 different inset between tall, steep slopes the creek is now piped species were noted during the (Figure 3.7). However, the underground in the North City Creek mapping work, with extent of vertical confinement Temple conduit. 50 of these species being native varies, and in some locations the to Utah. As seen in Table 3.3, channel shape is wider and Median (D50) streambed particle most of the nonnative species includes broader, flatter bank size at the measured cross within the corridor occur in the and floodplain surfaces (Figure sections ranges from 26 to 74 canopy and understory 3.7). These areas are important millimeters (mm), indicating that vegetation layers, while the shrub because they allow water to medium- and large-sized gravel layer is dominated almost spread out horizontally during are the dominant substrate sizes entirely by native species.
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Figure 3.7. Cross-section plots extrapolated from digital elevation data. Plots on left (in red) exhibit less vertical confinement and include relatively well-developed active floodplain surfaces. Plots on right (in blue) exhibit a greater degree of vertical confinement between tall, steep side slopes.
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Table 3.1. Summary of streambed material, channel geometry, and slope data. MEASURED VALUES AT RIFFLE CROSS SECTION REACH DATA STREAMBED MATERIAL SIZE DATA CHANNEL GEOMETRY REACH NUMBER Low Flow Wetted D16 D50 D84 Percent Local Slope Reach Slope Reach Length Wetted Width Width (ft) b (mm) a (mm) a (mm) a Embedded (ft/ft) d (ft/ft) d (ft) b (ft) b at 78 cfs c UCC_R09 8 26 99 12 7.0 21.0 0.008 0.029 1565 UCC_R10A 8 29 56 3 4.9 35.6 0.020 0.031 1427 UCC_R10B 17 39 102 22 4.4 13.4 0.014 0.036 1905 UCC_R10C 15 46 121 20 14.8 16.9 0.033 0.032 1612 UCC_R11A 17 43 148 29 13.6 16.6 0.024 0.033 1836 UCC_R11B 16 58 171 0 7.6 14.1 0.034 0.032 1207 UCC_R11C 14 40 79 9 13.4 19.7 0.015 0.038 1357 LCC_R01A 14 34 108 8 19.9 21.0 0.041 0.036 1686 LCC_R01B 32 74 187 7 10.9 13.7 0.031 0.045 836 LCC_R01C 15 38 128 13 19.0 23.2 0.028 0.035 1303 LCC_R01D02A 13 37 89 11 13.4 19.0 0.019 0.032 681 LCC_R02B - - - - 10.0 12.0 0.016 0.034 748 averages 15 42 117 12 11.7 19.5 0.024 0.034 1401 a The 16th, 50th, and 84th percentile values of the particle size distribution, in millimeters. b Feet. c Cubic feet per second. d Feet per feet.
Bigtooth maple (Acer chokecherry (Prunus virginiana) upper slopes. Nonnative turf grandidentatum), box elder (Acer and redosier dogwood (Cornus grass dominates the understory negundo), cottonwood (Populus sericea), with Wood’s rose (Rosa cover within the formal Memory sp.), and water birch (Betula woodsii) and skunkbush sumac Grove lawn area in reach occidentalis) are the most (Rhus trilobata) common on LCC_R02B. Common invasive common trees along the stream- upper portions of slopes. The understory species include lesser side areas of City Creek, with understory vegetation layer burdock (Arctium minus), Gambel oak (Quercus gambelii) shows the greatest species cheatgrass (Bromus tectorum), common on upper slope areas. variety. Creeping barberry and Dalmatian toadflax (Linaria Siberian elm (Ulmus pumila), an (Mahonia repens) and field dalmatica) (Table 3.3). introduced invasive tree, is also horsetail (Equisetum arvense) are common in the lower reaches of the most common native Near-stream canopy (tree) cover the study area and comprises the understory species in stream-side is generally high throughout the dominant canopy species in areas, with bluebunch study area, averaging 67% some reaches (Table 3.3). wheatgrass and ragweed (Table 3.4.). Tree cover is Common shrub species include (Ambrosia sp.) common on limited by the proximity of the
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Figure 3.8. Longitudinal profile plot of City Creek streambed based on 2006 digital elevation data. Black cross marks indicate culvert inlets and outlets; red and blue lines indicate open channel stream sections. canyon road in some areas, and Invasive weeds are a problem in issues are discussed by function is lacking in the formal Memory all study reaches, and 52% of the below. Grove lawn area (reach overall mapped area has a weed LCC_R02B). Because of the classification of moderate, high, Aesthetics generally high-quality tree cover or majority (Table 3.4). within the City Creek riparian Understory cover is minimal in Although many visually corridor, the riparian functions of the reaches downstream from appealing portions of City Creek shading and water-temperature reach LCC_R01A. exist, the presence of trash and control are met to a high degree. debris degrades corridor Thirty-seven percent of the Issues Affecting aesthetics in a number of overall mapped area has greater Riparian Functions locations. Common types of than 25% cover in all three trash include miscellaneous small vegetation layers (canopy, shrub, items such as bottles, cans, food During the baseline assessment understory); these areas are wrappers, ropes, tarps, etc. work, several common issues particularly effective at providing These items are common in the were observed to be affecting the riparian functions of nutrient more accessible stream reaches and limiting riparian functions in filtration and sediment trapping. that are used for recreation. the City Creek corridor. These
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Table 3.2. Plant species noted during City Creek mapping work. NATIVE TO UTAH SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS OR INTRODUCED Aibes concolor white fir not listed native Acer glabrum Rocky Mountain maple facultative native Acer grandidentatum bigtooth maple not listed native Acer negundo box elder facultative wetland native Aegilops cylindrica jointed goatgrass not listed introduced Aesculus hippocastanum horse chestnut not listed introduced Agrostis gigantea redtop no indicator introduced Ailanthus altissima tree of heaven no indicator introduced Alnus incana gray alder facultative wetland native Ambrosia artemisiifolia annual ragweed facultative upland native Artemisia cana silver sagebrush facultative native Arctium minus lesser burdock not listed introduced Artemisia tridentata big sagebrush not listed native Betula occidentalis water birch facultative wetland native Bromus tectorum cheatgrass not listed introduced Catabrosa aquatica water whorlgrass obligate wetland native Cardaria draba whitetop no indicator introduced Centaurea maculosa spotted knapweed not listed introduced Centaurea solstitialis yellow star-thistle not listed introduced Ceanothus velutinus snowbrush ceanothus not listed native Chenopodium album lambsquarters facultative upland native Chrysothamnus viscidiflorus yellow rabbitbrush not listed native Cirsium arvense Canada thistle facultative upland introduced Cichorium intybus chicory not listed introduced Cirsium vulgare bull thistle facultative introduced Cornus sericea redosier dogwood facultative wetland native Crataegus rivularis river hawthorn not listed native Cynoglossum officinale gypsyflower (houndstongue) not designated introduced Dactylis glomerata orchardgrass facultative upland introduced Elaeagnus angustifolia Russian olive facultative introduced Equisetum arvense field horsetail facultative native Euphorbia myrsinites myrtle spurge not listed introduced Fraxinus pennsylvanica green ash facultative wetland native Grindelia squarrosa curlycup gumweed facultative upland native Gutierrezia sarothrae broom snakeweed not listed native Helianthus annuus common sunflower facultative upland native Hedera helix English ivy not listed introduced Hordeum jubatum foxtail barley facultative native Isatis tinctoria Dyer's woad not listed introduced Juniperus osteosperma Utah juniper not listed native Lactuca serriola prickly lettuce facultative upland introduced
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Table 3.2. Plant species noted during City Creek mapping work (cont.). NATIVE TO UTAH SCIENTIFIC NAME COMMON NAME WETLAND INDICATOR STATUS OR INTRODUCED Linaria dalmatica dalmatian toadflax not listed introduced Lonicera involucrata twinberry honeysuckle facultative native Mahonia repens creeping barberry not listed native Maianthemum stellatum starry false lily of the valley facultative native Melilotus officinalis yellow sweetclover facultative upland introduced Medicago sativa alfalfa not listed introduced Onopordum acanthium scotch cottonthistle not listed introduced Parthenocissus quinquefolia Virginia creeper no occurrence native Phalaris arundinacea reed canarygrass obligate wetland native Phragmites australis common reed facultative wetland native Picea engelmannii Engelmann spruce facultative upland native Pinus nigra Austrian pine not listed introduced Pinus ponderosa ponderosa pine facultative upland native Picea pungens blue spruce facultative native Populus angustifolia narrowleaf cottonwood facultative native Populus deltoides eastern cottonwood facultative wetland native Poa pratensis Kentucky bluegrass facultative upland introduced Prunus virginiana chokecherry facultative upland native Pseudoroegneria spicata bluebunch wheatgrass obligate upland native Quercus gambelii Gambel oak not listed native Rhus glabra smooth sumac not listed native Rhus trilobata skunkbush sumac no indicator native Ribes montigenum gooseberry currant not listed native Robinia pseudoacacia black locust facultative upland native Rosa woodsii Woods' rose facultative native Rumex crispis curly dock facultative wetland introduced Salix bebbiana bebb willow facultative wetland native Salix exigua narrowleaf willow obligate wetland native Salix fragilis crack willow facultative introduced Schoenoplectus acutus hardstem bulrush obligate wetland native Scirpus microcarpus panicled bulrush obligate wetland native Secale cereale cereal rye not listed introduced Solanum dulcamara climbing nightshade facultative introduced Solidago missouriensis Missouri goldenrod not listed native Sorbus scopulina greene's mountain ash no indicator native Symphoricarpos albus common snowberry facultative upland native Symphyotrichum eatonii Eaton's aster facultative native Thinopyrum intermedium intermediate wheatgrass not listed introduced Toxicodendron rydbergii western poison ivy facultative upland native Tragopogon dubius yellow salsify not listed introduced Ulmus pumila Siberian elm not listed introduced
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Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach.
PLANT SPECIES UCC_R11C UCC_R11B UCC_R09 UCC_R11A LCC_R01C LCC_R01B LCC_R01A UCC_R10C UCC_R10B UCC_R10A Common Name Scientific Name LCC_R02B LCC_R01D_R02A White fir Aibes concolor XX Rocky mountain maple Acer glabrum X Bigtooth maple Acer grandidentatum XXXXXXXXXXXX Box elder Acer negundo XXXXXXXXXXXX Water birch Betula occidentalis XXXXXXXXX Green ash Fraxinus pennsylvanica XXXXXX Utah juniper Juniperus osteosperma XX Engelmann spruce Picea engelmannii XX Ponderosa pine Pinus ponderosa X Blue spruce Picea pungens XX Narrowleaf cottonwood Populus angustifolia XXXXXXXXXXXX CANOPY Eastern cottonwood Populus deltoides XXXXX Gambel oak Quercus gambelii XXXXXXXXXXXX Smooth sumac Rhus glabra X Austrian pine a Pinus nigra a X Horse chestnut a Aesculus hippocastanum a XXXX Russian olive b Elaeagnus angustifolia b XX XX Black locust c Robinia pseudoacacia c X Tree of heaven c Ailanthus altissima c XX Siberian elm b Ulmus pumila b XXXXXX Chokecherry Prunus virgniana XXXXXXX XXX Gray alder Alnus incana X Common snowberry Symphoricarpos albus X Snowbrush ceanothus Ceanothus velutinus XX Greene's mountain ash Sorbus scopulina X Redosier dogwood Cornus sericea XXXXXXXXXX River hawthorn Crataegus rivularis XX
SHRUB Twinberry honeysuckle Lonicera involucrata X Skunkbush sumac Rhus trilobata XXX XXXXXX Gooseberry currant Ribes montigenum XX Woods' rose Rosa woodsii XXXXXXXXXXX Bebb willow Salix bebbiana X Narrowleaf willow Salix exigua X XXXXX
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Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach (cont.).
PLANT SPECIES UCC_R11C UCC_R11B UCC_R09 UCC_R11A LCC_R01C LCC_R01B LCC_R01A UCC_R10C UCC_R10B UCC_R10A Common Name Scientific Name LCC_R02B LCC_R01D_R02A Chrysothamnus Yellow rabbitbrush XX X viscidiflorus Common reed Phragmites australis X Silver sagebrush Artemisia cana XXX SHRUB Big sagebrush Artemisia tridentata X Crack willow a Salix fragilis a XX XXX Annual ragweed Ambrosia artemisiifolia XXXXXXXXXXX Bluebunch wheatgrass Pseudoroegneria spicata XX XXXXXXXX Broom snakeweed Gutierrezia sarothrae XXX Common reed Phragmites australis XXX Common sunflower Helianthus annuus XXXXX X XX Creeping barberry Mahonia repens XXXXXXXXXXXX Curlycup gumweed Grindelia squarrosa XX XXXXXX Eaton's aster Symphyotrichum eatonii XXXXXXX X Field horsetail Equisetum arvense XXXXXXXX XX Foxtail barley Hordeum jubatum XXXX Hardstem bulrush Schoenoplectus acutus X Missouri goldenrod Solidago missouriensis XXXXXXX Panicled bulrush Scirpus microcarpus X Reed canarygrass Phalaris arundinacea XX Starry false lily of the valley Maianthemum stellatum XXXXXXXX
UNDERSTORY Parthenocissus Virginia creeper XXX quinquefolia Water whorlgrass Catabrosa aquatica XXX XX Western poison ivy Toxicodendron rydbergii XXXXXX Redtop a Agrostis gigantea a XX XXXXX X Yellow sweetclover a Melilotus officinalis a XXXX XXXXX Alfalfa a Medicago sativa a XXX Lambsquarters a Chenopodium album a X Orchardgrass a Dactylis glomerata a XXXXXX XXX X Kentucky bluegrass a Poa pratensis a XX X XX Intermediate wheatgrass a Thinopyrum intermedium a XX X Yellow salsify a Tragopogon dubius a X Chicory a Cichorium intybus a XXX
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-17 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Table 3.3. List of mapped canopy, shrub, and understory plant species found in each assessed stream reach (cont.).
PLANT SPECIES UCC_R11C UCC_R11B UCC_R09 UCC_R11A LCC_R01C LCC_R01B LCC_R01A UCC_R10C UCC_R10B UCC_R10A Common Name Scientific Name LCC_R02B LCC_R01D_R02A Climbing nightshade a Solanum dulcamara a XXXXXXXX Prickly lettuce a Lactuca serriola a XXXXXX XXX Curly dock a Rumex crispis a X Cereal rye a Secale cereale a XX Lesser burdock b Arctium minus b XXXXXXXX XX Cheatgrass c Bromus tectorum c XX X XX XXX Spotted knapweed b Centaurea maculosa b XX Yellow star-thistle b Centaurea solstitialis b XX Myrtle spurge b Euphorbia myrsinites b XXX English ivy c Hedera helix c XXX Dyer's woad b Isatis tinctoria b XXXXX UNDERSTORY Dalmatian toadflax b Linaria dalmatica b X XXXXXXXX Scotch cottonthistle b Onopordum acanthium b XX XXX Whitetop b Cardaria draba b X Gypsyflower Cynoglossum officinale b XXXX (houndstongue) b Jointed goatgrass b Aegylops cylindrica b XX Bull thistle b Cirsium vulgare b XX Canada thistle b Cirsium arvense b XX a Species not native to Utah. b State- or City-listed, nonnative, noxious weed species. c Nonnative, invasive species.
Another common category of include utility pipe crossings and a food source. Therefore, trash is remnant/obsolete pieces gabion basket bank protection. healthy native plant communities of infrastructure such as pieces of are necessary for a riparian concrete and asphalt or old pipes Wildlife Habitat corridor to function to its and barrels. In some instances and Connectivity maximum potential in terms of the concrete and asphalt pieces wildlife habitat. As discussed are associated with poorly A wide range of native bird and above, invasive nonnative plant designed bank stabilization mammal species rely on native species are a concern in all of the efforts. A total of 43 individual insects as a key food source study reaches within the City litter areas were mapped in the (Tallamy 2009). These insects Creek corridor, and significantly study area during the RCS must share an evolutionary affect the composition of the baseline assessment work. Other history with plants in order to understory and canopy items impacting aesthetics recognize them and use them as vegetation layers. In some areas
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-18 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Table 3.4. Percent cover and invasive species class for mapped vegetation polygons. POLYGON PERCENT CANOPY PERCENT PERCENT UNDERSTORY INVASIVE SPECIES REACH NUMBER COVER SHRUB COVER COVER CLASS UCC_R09 1 0 0 51—75 high UCC_R09 2 51—75 26—50 1—5 none UCC_R09 3 51—75 51—75 76—100+ low UCC_R09 4 0 6—25 76—100+ none UCC_R09 5 25—50 6—25 76—100+ none UCC_R09/10A 6 51—75 6—25 51—75 none UCC_R09 7 51—75 26—50 76—100+ high UCC_R09 8 76—100+ 6—25 76—100+ none UCC_R09 9 51—75 26—50 76—100+ moderate UCC_R09 10 76—100+ 26—50 26—50 low UCC_R09 11 51—75 26—50 51—75 moderate UCC_R09 12 6—25 1—5 51—75 majority UCC_R09 13 51—75 6—25 51—75 moderate UCC_R09 14 6—25 0 26—50 none UCC_R10A 15 51—75 6—25 51—75 moderate UCC_R10A 16 76—100+ 6—25 76—100+ low UCC_R10A 17 76—100+ 51—75 26—50 none UCC_R10A 18 51—75 6—25 26—50 low UCC_R10A 19 76—100+ 76—100+ 26—50 moderate LCC_R02B 20 26—50 6—25 51—75 majority LCC_R02B 21 26—50 26—50 76—100+ majority LCC_R01D/02A/02B 22 51—75 26—50 6—25 moderate LCC_R01D/02A/02B 23 26—50 26—50 51—75 moderate LCC_R02B 24 26—50 0 76—100+ moderate LCC_R01D/02A/02B 25 0—25 0—5 51—100+ low UCC_R10A 26 76—100+ 26—50 51—75 low UCC_R10A 27 76—100+ 51—75 26—50 low UCC_R10B 28 76—100+ 76—100+ 76—100+ none UCC_R10A/10B 29 76—100+ 51—75 51—75 moderate UCC_R10A/10B 30 51—75 76—100+ 6—25 none UCC_R10B 31 76—100+ 6—25 76—100+ low UCC_R10B 32 76—100+ 51—75 76—100+ none UCC_R10B 33 76—100+ 51—75 51—75 low UCC_R10B 34 76—100+ 26—50 76—100+ none UCC_R10B 35 76—100+ 26—50 76—100+ low UCC_R10B/10C 36 76—100+ 26—50 51—75 low UCC_R10B 37 76—100+ 26—50 76—100+ none UCC_R10C 38 76—100+ 26—50 26—50 low UCC_R10C 39 76—100+ 6—25 51—75 low UCC_R10C 40 76—100+ 26—50 76—100+ none
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-19 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Table 3.4. Percent cover and invasive species class for mapped vegetation polygons (cont.). POLYGON PERCENT CANOPY PERCENT PERCENT UNDERSTORY INVASIVE SPECIES REACH NUMBER COVER SHRUB COVER COVER CLASS UCC_R10C 41 76—100+ 6—25 51—75 low UCC_R10C 42 6—25 0 26—50 moderate UCC_R10C 43 51—75 26—50 51—75 low UCC_R10C 44 76—100+ 6—25 51—75 none UCC_R10C 45 76—100+ 26—50 51—75 none UCC_R10C 46 76—100+ 26—50 76—100+ none UCC_R10C 47 76—100+ 6—25 51—75 none UCC_R10C/11A 48 76—100+ 6—25 51—75 none UCC_R10C 49 76—100+ 51—75 51—75 low UCC_R10C 50 76—100+ 51—75 51—75 none LCC_R01B 51 51—75 26—50 6—25 none LCC_R01B 52 51—75 26—50 26—50 moderate LCC_R01B 53 6—25 0 76—100+ moderate LCC_R01B 54 51—75 6—25 26—50 low LCC_R01B/01C 55 51—75 6—25 26—50 low LCC_R01B/01C 56 76—100+ 6—25 6—25 moderate LCC_R01B 57 76—100+ 6—25 6—25 none LCC_R01B 58 6—25 1—5 51—75 high LCC_R01B 59 76—100+ 6—25 26—50 none UCC_R11A 60 51—75 51—75 76—100+ low UCC_R11A 61 51—75 26—50 76—100+ high UCC_R11A 62 51—75 26—50 26—50 moderate UCC_R11A 63 51—75 76—100+ 51—75 high UCC_R11A 64 51—75 26—50 26—50 high UCC_R11A 65 26—50 26—50 26—50 moderate UCC_R11A 66 76—100+ 51—75 51—75 low UCC_R11A 67 76—100+ 51—75 26—50 high UCC_R11A 68 51—75 26—50 51—75 low UCC_R11A 69 26—50 6—25 6—25 moderate UCC_R11A 70 51—75 51—75 51—75 low UCC_R11B 71 26—50 26—50 26—50 none UCC_R11B 72 76—100+ 26—50 26—50 none UCC_R11B 73 76—100+ 51—75 51—75 moderate UCC_R11B/11C 74 51—75 26—50 51—75 none UCC_R11B 75 51—75 51—75 51—75 moderate UCC_R11B 76 76—100+ 51—75 76—100+ none UCC_R11B 77 26—50 6—25 26—50 moderate UCC_R11B 78 26—50 51—75 26—50 moderate UCC_R11A/11B 79 51—75 6—25 26—50 moderate UCC_R11C 80 6—25 6—25 26—50 none
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-20 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Table 3.4. Percent cover and invasive species class for mapped vegetation polygons (cont.). POLYGON PERCENT CANOPY PERCENT PERCENT UNDERSTORY INVASIVE SPECIES REACH NUMBER COVER SHRUB COVER COVER CLASS UCC_R11C 81 51—75 26—50 6—25 none UCC_R11C 82 51—75 26—50 6—25 moderate UCC_R11C 83 26—50 6—25 26—50 moderate UCC_R11C 84 26—50 26—50 26—50 moderate UCC_R11C 85 51—75 26—50 26—50 moderate UCC_R11C 86 51—75 26—50 26—50 none UCC_R11C 87 26—50 6—25 26—50 high UCC_R11C 88 51—75 51—75 26—50 moderate UCC_R11C 89 51—75 26—50 51—75 high UCC_R11C 90 6—25 26—50 26—50 high UCC_R11C 91 76—100+ 51—75 6—25 moderate Lower Debris Basin 92 76—100+ 51—75 26—50 majority LCC_R01A 93 26—50 6—25 26—50 moderate LCC_R01A 94 51—75 6—25 26—50 high LCC_R01A 95 51—75 26—50 26—50 high LCC_R01A 96 26—50 6—25 6—25 moderate LCC_R01A 97 51—75 26—50 26—50 moderate LCC_R01A 98 51—75 26—50 26—50 high LCC_R01A 99 26—50 6—25 26—50 moderate LCC_R01A 100 76—100+ 26—50 26—50 moderate LCC_R01A 101 76—100+ 6—25 6—25 none LCC_R01A 102 51—75 26—50 6—25 moderate LCC_R01A/01B 103 51—75 26—50 26—50 none LCC_R01A 104 51—75 26—50 26—50 moderate LCC_R01B 105 26—50 6—25 26—50 moderate LCC_R01A 106 26—50 26—50 51—75 high LCC_R01A 107 6—25 6—25 26—50 moderate LCC_R01C 108 6—25 1—5 26—50 high LCC_R01C 109 6—25 1—5 51—75 high LCC_R01C 110 51—75 6—25 6—25 moderate LCC_R01C 111 6—25 6—25 51—75 high LCC_R01C 112 26—50 6—25 51—75 high LCC_R01C/01D/02A 113 6—25 6—25 76—100+ moderate LCC_R01C 114 76—100+ 6—25 0 high LCC_R01C/01D/02A 115 76—100+ 6—25 51—75 high LCC_R01C 116 26—50 6—25 51—75 moderate LCC_R01C 117 76—100+ 26—50 6—25 high LCC_R01D/02A 118 26—50 0 51—75 high LCC_R01D/02A 119 51—75 51—75 51—75 high LCC_R01D/02A 120 76—100+ 51—75 6—25 moderate LCC_R01D/02A 121 51—75 26—50 51—75 high LCC_R01D/02A 122 51—75 6—25 51—75 majority
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-21 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
particularly important for bird Dominant canopy species/ populations. Invasive plants of concern vegetation types in the in the study area: study area: As discussed previously, the longitudinal connectivity of City • spotted knapweed • bigtooth maple Creek within the RCS study area is relatively intact compared to • yellow starthistle • bigtooth maple-Gambel other streams within Salt Lake oak City. However, the debris basins • dalmation toadflax and associated culverts near • box elder Bonneville Boulevard create an • dyer’s woad interruption in the corridor that •Gambel oak impedes fish passage (Figure • whitetop 3.12, Table 3.5). The other • introduced/native tree mix crossings within the study area • Canada thistle are generally footbridges that do •gypsyflower •ornamental mix not significantly impede (houndstongue) connectivity with regards to fish • other native trees or or wildlife (Table 3.5). However, shrubs • myrtle spurge several concrete in-channel • Siberian elm apron/sill structures that protect • Scotch thistle pipe crossings create significant • understory vertical drops that may hinder • bull thistle fish passage (Table 3.6). • water birch • common burdock Nutrient Filtration and Sediment Trapping • Russian olive
Several areas within the City • Siberian elm Creek corridor lack the dense near-stream understory and • tree of heaven shrub cover that is needed to • jointed goatgrass maximize the ability of the riparian corridor to filter • English ivy sediment, nutrients, and pollutants from storm runoff. • black locust invasive species comprise the Another factor limiting the majority plant cover within a filtration function in portions of • cheatgrass vegetative layer, limiting the City Creek is the design of storm ability of native plants to thrive drain outfalls. Some pipes and support native insects, birds, discharge directly into the main and wildlife. A lack of understory flow of the stream, eliminating and shrub cover in some reaches the opportunity for runoff to be also limits habitat quality in terms filtered by riparian vegetation. of structural diversity, which is
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-22 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Table 3.5. Bridges and culvert crossings in the study area. APPROXIMATE CROSSING REACH CULVERT CROSSING SIZE/ LOCATION/ CONDITION RECOMMENDATIONS NUMBER(S) LENGTH TYPE DESCRIPTION DESCRIPTION (feet)
between UCC_R10B pre-fabricated full span, high Red Footbridge N/A a good none and UCC_10C metal footbridge clearance
wood plank on low clearance; does remove or repair to Wood Plank between UCC_11B and planks missing; N/A a metal I-beam not span entire eliminate safety Footbridge UCC_11C in disrepair supports floodplain concern
between UCC_R11C Bonneville concrete culvert and lower debris 96 6-foot diameter good none Boulevard pipe basin
between lower debris concrete culvert Canyon Road 72 6-foot diameter good none basin and LCC_R01A pipe
pre-fabricated full span, high Brown Footbridge middle of LCC_R01B N/A a good none metal footbridge clearance
high clearance, between LCC_R01B footbridge across spans width improve bed protection Stream Gage N/A a good and LCC_R01C concrete flume between concrete below concrete flume flume walls
evidence of monitor condition; if Concrete utility concrete/stone 6-foot-wide arch; sediment LCC_R01C 6 feasible replace with crossing arch culvert 3.5-foot clearance deposition on full-span structure upstream side
monitor condition; if Concrete utility concrete/ stone 7-foot-wide arch; fair; crack in LCC_R01D_02A 6 feasible replace with crossing arch culvert 3.5-foot clearance cement full-span structure
3-foot clearance; remove bridge/wingwalls Concrete concrete bridge spans width fair; some scour and replace with bridge LCC_R01D_02A N/A a Footbridge and wingwalls between concrete at wingwalls that spans entire wingwalls floodplain
arched rockwork full span, high Rockwork Bridge middle of LCC_R02B N/A a good none footbridge clearance
high clearance; Concrete Bridge in flat concrete spans width Memory Grove LCC_R02B N/A a footbridge with good none between grouted Park metal railing rock walls a Not applicable.
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Table 3.6. Existing significant streambed protection structures in the study area. ESTIMATED VERTICAL POTENTIAL FISH DROP IN WATER LATERAL CHANNEL REACH a DESCRIPTION BARRIER SURFACE AT CONFINEMENT STRUCTURE b
UCC_R11C concrete sill/old gage yes 3 feet yes (concrete wall)
UCC_R11C concrete and rubble yes two drops, each 2 feet yes (concrete walls)
UCC_R11C narrow concrete sill no 0.5 foot yes (concrete piers)
yes (concrete wing walls; LCC_R01B narrow concrete sill no 0.8 foot gabions)
LCC_R01B cemented bed yes 2.5 to 3 feet no
LCC_R01B boulder weir unlikely 2 feet total no
not confined at structure, concrete block, boulders, LCC_R01B unlikely several drops, each 2 feet but confined just upstream rubble by stone block wall
LCC_R01B two long concrete pieces no 0.5 foot no
concrete flume LCC_R01B yes two drops; 3.5 and 2.5 feet yes (concrete walls) (stream gage) grouted rock drop yes (grouted rock LCC_R01C yes 4 feet structure wing walls) grouted rock drop yes (narrow grouted rock LCC_R01D_02A yes 10 feet structure wing walls) grouted rock drop yes (grouted rock wing LCC_R01D_02A yes 3.5 feet structure walls) grouted rock drop LCC_R02B yes 5 feet yes (grouted rock channel) structure grouted rock drop LCC_R02B yes 5 feet yes (grouted rock channel) structure grouted rock drop LCC_R02B yes 5 feet yes (grouted rock channel) structure a Structures listed in upstream to downstream order. b Assessed at low flow.
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-24 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Factors limiting shrub and understory cover:
• oversteepened slopes
• inadequate revegetation efforts following maintenance or construction work
• soil compaction from heavy foot traffic
• poorly controlled runoff from upland areas
Stream Stability
A number of different issues were noted as affecting stream stability within the City Creek riparian corridor. Specific issues are discussed in the subsections Top left: Steep bare slope adjacent to Bonneville Boulevard in reach below. LCC_R01A. Note trees, grass, and large rocks at base of slope indicating toe stability. Top right: Rill erosion below pipe outfalls. Bottom left: Rill erosion where road runoff concentrates at gabion/ Fill Encroachment natural bank interface. Bottom right: Bare bank and compacted soil Steep, poorly vegetated fill slopes associated with heavy recreational foot traffic. associated with roads affect stability in portions of the lower reaches of City Creek and are Road Runoff and Storm flows under roads and into City particularly prevalent in reach Drain/Pipe Outfalls Creek. The outfalls lack LCC_R01A. Because these Erosion associated with poorly adequate outlet protection to slopes lack vegetation and the controlled road runoff was dissipate runoff velocities and associated resistive strength it observed in several study lack stabilized conveyance provides, they are vulnerable to reaches. In a number of channels between the outlet and erosion during flood events. Fill instances, road runoff the main City Creek channel. slopes also restrict floodplain concentrates at the edges of Clogged roadside storm drain width and laterally confine high installed gabion basket bank inlet grates were also observed in flows, causing increased flow armoring, causing rill several locations, and could lead velocities and shear stress on development at the gabion/ to further rill creation when flows streambanks. Fill encroachment natural bank interface. Rills were bypass the designed storm drain also limits the ability of floodplain also observed at several pipe system. surfaces to act as sponges to unprotected pipe outfalls that absorb flood water. convey tributary and storm
SALT LAKE CITY OCTOBER 2010 DEPARTMENT OF PUBLIC UTILITIES3-25 BIO-WEST, INC. RIPARIAN CORRIDOR STUDY FINAL CITY CREEK MANAGEMENT PLAN
Bank Erosion flashy, frequent, erosive runoff Lateral erosion of streambanks is events during storms. Because a natural process in stream the City Creek watershed channels, which are dynamic remains relatively natural, systems. Erosion and sediment evidence of this type of erosion is transport are necessary for the minimal in the study area. The creation and maintenance of fact that nearly 1 mile of bank important habitat features such length in the study area has been as scour pools, undercut banks, artificially hardened with gabion and spawning gravels. baskets and other structures also Deposition of sediment on limits the potential for this type of floodplain areas is also bank erosion. As discussed important, as it provides fresh previously, in the areas where substrate for the growth of willow gabions were installed after the and cottonwood seedlings that 1983 flood, the present-day are needed to maintain native streambed generally remains Above: Steep vertical bank riparian forests. However, flush with the lowest tier of where stream has migrated excessive amounts of erosion or gabions, suggesting that bed laterally into a fine-grained deposition can degrade habitat elevations have remained stable geologic formation. and water quality and threaten since the 1980s. On other infrastructure and homes. streams assessed as part of the Compaction from Foot RCS, bed lowering and Traffic In some reaches of City Creek associated bank erosion caused Within the downstream reaches tall, vertical, bare banks are by the toe of the slope being of the study area that receive the present where the creek has undermined was a commonly greatest recreational use, migrated laterally into steep, fine- observed process. This does not trampled bank and floodplain grained alluvial or tuff appear to be occurring to a areas were commonly observed. formations. This is a natural significant degree within City Steep, poorly stabilized access erosion process that is relatively Creek. trails also occur locally common within confined canyon throughout the study area and settings where less-resistant are commonly associated with geologic formations are present. established picnic sites. These Low bank/root zone erosion is areas lack vegetation and are another type of bank erosion that susceptible to erosion during is common in urbanized channels storm and flood events. where hydrology is dominated by
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