Assessing Riparian Canopy

The Tryon Creek Highway 43 Culvert Replacement Project, scheduled to begin work in 2025, will open passage from the Willamette river to Tryon Creek for salmonid species of fish to travel upstream into the watershed.

The current culvert obstructs fish passage under most conditions.

A wider, natural-bottom culvert means the Tryon Creek watershed could once again become home to migratory fish such as trout and salmon.

Stream temperature is a major concern for fish habitat suitability; according to Washington state's 2023 State of Salmon in Watersheds Report,

water temperatures greater than 64 degrees Fahrenheit stress salmon, and temperatures above 70 degrees Fahrenheit can be lethal.

One way to safeguard streams against high temperatures is to ensure adequate shading is provided by the trees in the riparian zone surrounding the creek.

The goal of this project is to identify areas within the watershed that may expose Tryon Creek and its tributaries to warming due to a lack of adequate shade.

With these areas identified, future restoration work can be better informed about where to focus valuable time and effort.

The bulk of the analysis was done with Lidar data from 2019: One raster layer containing All Lidar points, known as a Digital Surface Model (DSM), and another containing only points classified as Ground, commonly called a Digital Elevation Model (DEM).

This Lidar data was downloaded from the NOAA Data Access Portal; this data was provided by the Oregon Lidar Consortium (OLC), the U.S. Geological Survey (USGS), and the Oregon Department of Geology and Mineral Industries (DOGAMI).

The stream layer was downloaded from Oregon Metro's RLIS Discovery portal.

The watershed boundary layer was downloaded from Tryon Creek Watershed Council datasets.

Water temperature data was downloaded from the Aquarius data portal provided by the City of Portland's Environmental Services division.

Field data was collected on May 14th in Tryon Creek State Park; ArcGIS Field Maps and a spherical densiometer were used to estimate and record canopy cover percentage at several different spots directly over the creek.

This data was imported as points to provide some ground truthing to the canopy density analysis.

All analysis was performed in ArcGIS Pro.

My analysis began with basic geoprocessing steps, such as making sure all data were in the same map projection and clipped to the watershed boundary.

Then, I defined the riparian zone by adding a 75 foot buffer on either side of the streams.

The stream layer and resulting riparian buffer is disjointed to reflect sections of stream that are piped underground.

My next step was to determine canopy density throughout the watershed, in order to identify areas where canopy cover may be less dense, providing less shade over the creek.

This required some simple "map math" using the Raster Calculator tool, and the two Lidar layers, the Digital Surface Model and the Digital Elevation Model.

First, I subtracted the DEM layer from the DSM layer, to arrive at the resulting Feature Heights layer.

Then, I divided the Feature Heights layer by the DEM layer to estimate the density of the tree canopy.

This canopy density layer was verified with the canopy cover points collected in the field.

The next part of the analysis involved calculating the amount of incoming sunlight throughout the riparian zone.

Taking the DEM layer, I used the Surface Parameters tool to derive Slope and Aspect layers to be used in the next process.

I ran the Raster Solar Radiation tool, which calculates the amount of solar insolation for every raster cell within the riparian buffer zone. Inputs were the Feature Heights layer, as well as Slope and Aspect layers. The time interval specified was May 1, 2023 - September 1, 2023 to encapsulate the hottest and sunniest time of the year.

To better visualize the areas where incoming solar radiation is high and and canopy density is low, I overlaid the canopy density with the solar radiation results, and made the areas with little to no canopy transparent to allow the solar radiation layer to peek through.

Finally, water temperature points were created from seven data collection locations that had sufficient recent summer records.

The average water temperature was calculated using hourly measurements for the months of July and August, and symbolized in graduated colors showing hotter points as darker orange than cooler points.

The analysis resulted in multiple areas of interest; headwaters in the western portion of the watershed generally have less canopy cover, and more exposure to sunlight due to development.

Streams that have north/south orientations, or are situated on south-facing slopes tend to be exposed to more sunlight overall.

These factors are known to impact stream temperature; with that in mind, there are some areas of particular interest with regards to shading and mitigating high creek temperatures that the Watershed Council might want to delve into further.

There are several areas along the main stem of Tryon Creek and within the state park that feature little canopy cover, and relatively high incoming sunlight, that could represent opportunities for significant water warming.

The first location is not technically in the boundary of the state natural area, but just north of it. The average stream temperature recorded here (for the months of July and August, 2021-2023) is very concerning; some days exceeded temperatures of 22 degrees Celsius, or 71.6 degrees Fahrenheit, which is lethal for salmon.

The next area is just downstream, with open canopy that may expose the already warm water to more sun.

The third location is a spot where the creek meanders, likely allowing the water to slow down--this, combined with the open canopy, shows potential for warming the creek a significant amount.

There are also several areas of interest in the headwaters of the watershed that could warrant further investigation.

The stream temperature of 17.5 degrees Celsius near the confluence of Falling Creek and Tryon Creek, along with the lack of dense canopy in the northernmost part of the watershed indicates there could be an opportunity for improvement here.

This property on Englewood Drive seems to have a large spring. It is unclear if this potential spring still feeds a tributary of the creek; the area immediately downstream is entirely unshaded.

This area just north of Lake Oswego High School also may not contribute much water volume to the creek, but the analysis shows its canopy cover could be improved.

Watersheds are very complex systems with a large number of factors that influence a stream's habitability for sensitive fish species.

I learned a lot about hydrology, freshwater habitat suitability and water infrastructure management through the course of this project; enough to recognize that this topic is much more complicated than I could have imagined at first.

These results are not intended to represent a straightforward or comprehensive answer to a question, but rather a path to asking deeper, better-informed questions about how we can best support and manage the health of the watershed, for current residents and future residents as well.