By Anna Willig and Lauren McGrath | Willistown Conservation Trust Watershed Protection Program
Cover Photo by the Watershed Protection Team
Since 2018, the Watershed Protection Program has monitored water quality at ten sample sites in the headwaters of Darby, Crum, and Ridley Creeks (Map 1). Every four weeks, the team visited each of the ten sites to take in-stream measurements and collect samples for analysis in the lab. We are proud to present our findings on water quality based on analysis of our data collected from 2018 through 2021, which includes 41 monitoring visits and over 7500 different measurements.
August is National Water Quality month, and each week we will publish excerpts from one chapter from our report. Last week, Chapter 1 we focused on the basic water chemistry parameters of water temperature, dissolved oxygen, and pH. This week, we are focusing on the physical characteristics of the stream: discharge, turbidity, and total suspended solids.
The full report, which includes more information than is provided in the blog posts, can be found here.
Monitoring the physical characteristics of streams, in addition to water chemistry, provides deeper insight into water quality and stream health. Discharge is the volume of water flowing through the stream, turbidity is the cloudiness of water, and total suspended solids is a measurement of the total mass of sediment and debris in the water.
These parameters are influenced by the landscape in the surrounding watershed. Impervious surfaces, or surfaces that block the infiltration of water into the soil, can increase the velocity and energy of water during storm events, allowing stormwater to pick up and carry more sediment into the nearest body of water. By preventing infiltration, impervious surfaces also force more water into streams during storm events, resulting in greater flooding and erosion. The removal of trees and shrubs along waterways destabilizes soil, leading to more sediment in streams. Headwater streams, which are the origins of stream systems, can carry tons of sediment and pollution into the stream throughout the entire waterway, harming downstream ecosystems. Studying discharge and sediment dynamics can help identify areas of rapid erosion that should be targeted for restoration.
The headwaters of Darby, Crum, and Ridley Creeks are highly responsive to rainfall events, as indicated by spikes in discharge after storm events. There is constant, though often low, movement of sediment through the sample sites, as indicated by turbidity and total suspended solids. Sediment movement increases after rainfall or snowmelt events. Though there are no regulations governing acceptable amounts of sediment in streams, it is possible that sediment poses a risk to stream life during storm events. Analysis of sediment movement and erosion dynamics is ongoing to identify areas where restoration should target soil stabilization.
For a primer on statistical tests and how to read boxplots and scatterplots, click here.
Figure 1. Stream discharge from January 2018 through December 2021 (a) across ten sample sites in the headwaters of the Darby, Crum, and Ridley Creeks and (b) over time.
Discharge is the volume of water flowing through a stream and is measured in cubic meters per second (m3/s). Discharge reflects the size of the stream; as more tributaries enter the stream and it gets bigger, discharge increases as well. Prior to June 5, 2019, discharge was not consistently measured on sample days.
There are significant differences in discharge between sites. Within each watershed, discharge is significantly higher at the most downstream site than it is at the most upstream sites, as expected, and discharge at Ridley Creek State Park (RCSP1) is significantly greater than discharge at all other sample sites (Figure 1a).
Spikes in discharge are related to rainfall or snowmelt events, which increase the amount of water flowing through the stream. Changes can also be caused by debris in the stream channel. For example, the abrupt increase in discharge at RCSP1 on September 25, 2019 was likely caused by a downed tree that dammed Ridley Creek just downstream of the sample site (Figure 1b). By January 8, 2020, the tree was cleared and discharge returned to normal (Figure 1b). Decreases in discharge during summer months are generally caused by lack of rainfall. The lowest discharge at all sites but Crum Creek Main Stem Downstream (CC3) was recorded on August 16, 2021, after an extended period with little rain (Figure 1b).
Figure 2. Turbidity from January 2018 through December 2021 (a) across ten sample sites in the headwaters of the Darby, Crum, and Ridley Creeks and (b) over time.
Turbidity measures the amount of light that can pass through a water sample. Clear water has low turbidity and cloudy water has high turbidity. Turbidity provides an estimate for the amount of sand, silt, and other sediment in the water; as the amount of sediment in the water increases, so does the turbidity. However, turbidity is sensitive to the type of sediment — differences in the size, texture, and shape of sediment particles impact turbidity measurements — and should be considered a gross approximation for sediment concentration.
There are no significant differences in turbidity between sites (Figure 2a). Turbidity does not vary seasonally (Figure 2b). There is a significant but weak correlation between turbidity and discharge, suggesting that spikes in turbidity are related to influxes of sediment after rainfall or snowmelt events. The weakness of the relationship is likely due to differences in the amount of erosion and the type of sediment at each sample site.
Total Suspended Solids
Figure 3. Total suspended solids from January 2018 through December 2021 (a) across ten sample sites in the headwaters of the Darby, Crum, and Ridley Creeks and (b) over time. (c) The relationship between turbidity and total suspended solids.The blue line is the linear trendline and the shaded region represents the 95% confidence interval.
Closely related to turbidity is total suspended solids, or the mass of suspended particles, usually sediment, in a specified volume of water. While turbidity roughly estimates the amount of sediment in the water, total suspended solids is a more exact approximation. Though the transport of sediment is a natural process in streams and rivers, excess or sudden movement of sediment can harm aquatic organisms. When sediment is deposited on stream beds, it can smother macroinvertebrates and cover crucial streambed habitat. In the water column, suspended solids absorb sunlight, heating up the water and limiting the ability of aquatic plants and algae to photosynthesize. Though total suspended solids analysis helps understand erosion and sediment movement in waterways, there are no state or federal standards governing acceptable concentrations of total suspended solids.
There are no significant differences in total suspended solids between sites (Figure 3a). total suspended solids does not show strong seasonal variation; spikes in total suspended solids are likely due to influxes of sediment after precipitation events (Figure 3b). There is a significant, but weak, correlation between discharge and total suspended solids. There is a significant correlation between turbidity and total suspended solids, indicating that turbidity can reflect total suspended solids (Figure 3c). However, due to differences in sediment characteristics, turbidity should only be used to predict total suspended solids at a site-specific scale.
Future analysis includes developing a rating curve to estimate the amount of sediment moving through a stream, which will help identify areas of rapid erosion that should be targeted for restoration.
- Discharge generally remains constant, but is responsive to rainfall events.
- There is constant, though often low, movement of sediment through our streams.
- Storm events can wash large quantities of sediment into streams, potentially posing a risk to aquatic organisms.
- To learn more about discharge, erosion, and flooding, check out Flooding 101 and Flooding 102.
To read the full “State of our Streams Report,” click here.
Map 1. Willistown Conservation Trust’s sampling sites. Five sample locations are within the Ridley Creek watershed, four are within the Crum Creek Watershed, and one is within the Darby Creek Watershed. Sampling was conducted at each site every four weeks from January 2018 through December 2021.
This report was made possible through a grant from the William Penn Foundation. The WIlliam Penn Foundation, founded in 1945 by Otto and Phoebe Haas, is dedicated to improving the quality of life in the Greater Philadelphia region through efforts that increase educational opportunities for children from low-income families, ensure a sustainable environment, foster creativity that enhances civic life, and advance philanthropy in the Philadelphia region. In 2021, the Foundation will grant more than $117 million to support vital efforts in the region.
The opinions expressed in this report are those of the author(s) and do not necessarily reflect the views of the William Penn Foundation.
— By Anna Willig and Lauren McGrath | Willistown Conservation Trust Watershed Protection Program