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Understanding the Relationship Between Urban Trees, Stormwater Runoff, and Cold-Water Streams in a Changing Climate

Managing urban tree canopy to mitigate ecological drought and urban thermal pollution above salmonid streams from concurrent climate change and urbanization
Principal Investigator
Kevan Moffett


Start Date
End Date
Release Date


In the Pacific Northwest, cold-water species like salmon are important for recreational sport fishing as well as for commercial fish production. However, climate change is causing lower and warmer summer stream flows that could decimate these fish populations. Aquatic cold-water habitats are further threatened by stormwater runoff, which moves from streets to storm drains and then is often discharged into cold-water rivers and streams. The urban heat island effect (when cities and urban areas are warmer than surrounding rural areas) may heat the temperature of the runoff and if the runoff is very warm, it could tip salmon habitat into fatal conditions. However, specifically how urban warming – or cooling, such as by tree shade – affects [...]

Child Items (3)


Principal Investigator :
Kevan Moffett
Funding Agency :
Northwest CSC
CMS Group :
Climate Adaptation Science Centers (CASC) Program

Attached Files

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“Portland, Oregon - Credit: Alan Cressler”
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“Lauren Burns conducting field work”
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“Lauren Burns conducting field work”
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“Lauren Burns conducting field work”
thumbnail 2.53 MB
“Lauren Burns conducting field work”
thumbnail 2.32 MB


Climate change in the Pacific Northwest is causing lower and warmer summer stream flows that can decimate cold-water species like salmon. Impacts are particularly strong in cities, where the urban heat island can raise stream temperatures even within greenspaces. However, specifically how urban warming – or cooling, such as by tree shade – affects the temperature of stormwater runoff from streets to storm drains is surprisingly sparsely documented. This is important because storm drains of many cities in the Pacific Northwest discharge directly to cold-water aquatic ecosystems. If the runoff is very warm, it could tip salmon habitat into fatal conditions. On the other hand, urban street trees are already well known, and even managed, to help trap rainfall and reduce runoff quantity. This study will test if street trees could also be managed to help reduce runoff temperature. Data from streets in Portland, OR, will document how runoff temperature is affected by the amount and type (deciduous vs. conifer) of street trees in different locations and seasons over two years. The data will then be used to improve models of street tree benefits that already widely help plan cities’ tree planting and management, but which do not yet consider runoff temperature. This project will also analyze existing databases of street trees in cities throughout the Pacific Northwest to determine if some types of trees are less healthy than others amid their urban heat island, and assess whether some tree types might be better investments given pending climate warming. The results of this project will provide missing data and decision-support information to cities throughout the region, on how to use existing street tree “infrastructure” to, at little additional cost, also help reduce the negative impacts that warm urban runoff can have on aquatic ecosystems already stressed by drought and climate change.

Project Extension

typeTechnical Summary
valueObjectives/Justification. This study will test whether explicitly managing the urban forest canopy to reduce street runoff temperatures (TR) is an innovative and practical tool to mitigate urban contributions to urban and downstream ecological drought. This particularly innovative idea to help address ecological drought would leverage the substantial, highly managed urban forest “capital” to simply address an additional variable, TR, at little additional management cost. It would also be both mitigative of ecological drought and adaptive to climate change (as some aspects of relative cooling increase with energy load). Background. While the urban heat island is known to raise stream temperatures even within greenspaces, data are lacking on street-level TR and its relationship to street tree canopy (STC). This is important because curbside storm drain systems of many cities in the Pacific Northwest discharge directly to cold-water aquatic ecosystems. Large quantities of rapid runoff already can create impairment in receiving waters, and elevated runoff temperature could duplicate or exacerbate climate-driven aquatic system ecological drought. Methods. There are 4 specific objectives of this study. [1] Empirically quantify the effects of STC cover and tree type on street runoff temperature (TR) by 2 years of original field monitoring of TR within a 2x2 factorial experimental design (high/low STC cover vs. deciduous/coniferous tree types), throughout the range of weather and phenology in Portland, OR, as a pilot case study. [2] Develop a physics-based model based on 1D heat transport in a stream, but adapted to rapid, shallow stormwater flow over pavement and under intermittent shade, to predict spatially-explicit street gutter TR given variable STC, urbanization, and climate influences. [3] Predict urban tree types more likely to be healthy and beneficial (i.e., good investments) for mitigating downstream ecological drought under future climate warming stress by statistically analyzing the health of street trees in recent inventories across the PNW (≥8 cities, ≥334,000 trees) compared to the relative warmth of their location within the local urban heat island. [4] Compare the field and modeling with the USDA Forest Service urban forest decision-support model suite i-Tree, recommend i-Tree updates to represent currently missing TR and its relationships to STC, and add to the i-Tree Hydro module a means to assess potential TR influences on downstream ecological drought. Products, Partners, and Information Transfer. The field data, statistical analyses, and numerical models will be collaboratively reviewed and shared with a partner in Portland Environmental Services leading the street tree planting and education program and the originators of the PNW-wide city street tree inventories, presented at public (seminar, education, and outreach activities) and academic forums, documented in peer-reviewed manuscripts, and made publically available as per the Data Management Plan.
projectStatusIn Progress

Budget Extension

typeAward Type
valueCooperative Agreement
typeAward Number (FY16)
typeAward Number (FY17)

Portland, Oregon - Credit: Alan Cressler
Portland, Oregon - Credit: Alan Cressler


Spatial Services

ScienceBase WMS


  • National and Regional Climate Adaptation Science Centers
  • Northwest CASC



Additional Information


Type Scheme Key
RegistrationUUID NCCWSC e9f7dac7-f024-4fa9-b6dc-213a1ab6409b
StampID NCCWSC NW16-MK0567

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