Beavers are Cleaning Stormwater, Cooling Streams, and Increasing Complexity in Gresham
Katie Holzer Watershed Scientist City of Gresham, Oregon 1 Overview 1) Stormwater facility study 2) Stream temperature study 3) Stream complexity observations
2 Gresham
3 Dozens of Beaver Dams in Gresham Streams
• Seem to be increasing drastically in past 10 years • Most strongly associated with public land
4 Variety of Dams
https://www.facebook.com/JohnsonCreekWC/videos/382315012475862/ 5 1) Stormwater Facility Study
6 Columbia Slough Regional Water Quality Facility • Constructed in 2007-08 • 13-acre site • Treats 965 acres of industrial and commercial land • Cost = $2.4M • Goals: clean stormwater, provide habitat, foster education
7 Columbia Slough Regional Water Quality Facility
8 Columbia Slough Regional Water Quality Facility + Beavers
9 Question: Do the beaver dams help or hinder the water quality treatment in this facility?
10 Methods – Dam Removal
11 Methods – Water Quality Sampling
• Collected water quality samples during storms • Inlets and outlets of facility • Before and after dam removal and rebuilding • 7 storms without dams, 7 storms with dams • Metals, nutrients, sediment, pesticides
12 Results 100 No beaver dams 80
60
40
20
% Pollutant Removal Pollutant % 0
-20
-40
13 Results 100 No beaver dams With beaver dams 80
60
40
20
% Pollutant Removal Pollutant % 0
-20
-40
14 Beaver dams slow and filter stormwater
15 New Question: What if the beavers leave?!
16 Continually Remove One Dam
17 Suggestions for Designing Stormwater Facilities with Beaver in Mind • Always consider what it might look like if beavers show up (or not) • If you don’t want them • Try to minimize constrictions with running water • Avoid beaver food • If you do want them • Allow space for extra ponding • Provide food • Be open to change
18 2) Stream Temperature Study
19 Heat as a Major Pollutant in Johnson Creek
• Major causes Dead chinook Salmon in • Lack of shade Johnson Creek during hot June • Dam/ponds 2015
Photo by Danielle Miles
1 Temperature Study - Methods • Human and beaver dams throughout Gresham • >2’ deep • Maintained throughout the year • 6 of each type
Downstream loggers
Human dam
Beaver dam
Upstream loggers
21 Study Sites
Dam Type Human Beaver
5 Temperature Study - Results
Human dam B Human dam C 27 Human dam A 24 21 18 15 Temperature (*C) Temperature 12 Second week in June 2016 Upstream of pond Downstream of pond Not to exceed water quality standard
27 Beaver dam A Beaver dam B Beaver dam C 24 21 18
Temperature (*C) Temperature 15
12 23 Change in Maximum Stream Added Temperature Exceedances Temperature Downstream of Downstream of Dam Dam 80 5 70
4 60 50 3 40 Days 2 30
* Celsius 20 1 10
0 0 Human Beaver Human Beaver -10 -1
14 Water seeps through the beaver dams and pulls water from the entire stratified water column
25 How has this Affected our Management? • Worry less about added heat in Gresham • Allow dams to persist when not threatening infrastructure
26 Very Different Results on Errol Creek in Portland
27 Factors that likely affect temperature changes from beaver dams • Upstream temperature • Shade • Surface area • Depth • Connection to groundwater
28 3) Stream Complexity Observations
29 Many of Gresham’s Urban Stream Channels Have Low Complexity
30 Finding Sediment-Free Rocks Immediately Downstream of Dams
31 Starting to Find Gravel Bars Downstream of Dams
32 New Grassy Bar Below 3-Year Old Beaver Dam
Beaver dam
New bar
33 4’ pool Downstream of Dam in Otherwise 1’ Creek
34 Dam Causing Water to Find New Course Through Gopher Holes
35 New Side Channel in Beaver Creek within One Year of New Beaver Dam
Breached new beaver dam
New side channel 36 More Diverse Macroinvertebrate Communities?
Field duplicate immediately downstream of beaver dams (B-IBI = 30) had more diversity, fewer snails, and more mayflies and caddisflies than duplicate further away from dams (B-IBI = 28)
37 Starting to see some of this!
38 Overview • Beavers help clean stormwater in a large constructed facility • Beaver dams retain or cool stream temperatures on mainstem Johnson and Beaver Creeks in Gresham • Beaver activity is increasing complexity in Gresham streams • City is attempting to allow and support beavers when infrastructure is not threatened
39 Questions?
40