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A Macroinvertebrate’s Thanksgiving Feast

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Each fall, trees provide a veritable Thanksgiving Feast for aquatic macroinvertebrates. They spend the whole spring and summer preparing this feast, growing it, maintaining it, making sure everything is just right, and, come fall, dropping it all in the stream. What is this feast? Leaves!

Leaves from trees and shrubs are a form of allochthonous input, or something that enters an ecosystem from outside of the system. In small headwater streams, such as those found in our area, allochthonous input is a dominant source of energy, essentially forming the base of the food chain. Generally, these small headwater streams are forested, preventing light from reaching streams, inhibiting photosynthesis, and thus limiting the growth of aquatic plants. Consequently, the main source of energy in these streams comes from trees in the form of fallen leaves.

Once leaves fall into the stream, microbes, such as bacteria and fungi, colonize leaves, starting the process of decomposition. Some macroinvertebrates, called shredders, eat the leaves themselves, feasting on the plant material. Other macroinvertebrates, called scrapers, eat the layer of algae, bacteria, and fungi that develops on fallen leaves. Both shredders and scrapers are eaten by predatory macroinvertebrates, which are in turn eaten by frogs, salamanders, fish, and other aquatic animals, moving energy up the food chain.  

Leaf litter in streams also provides important habitat for macroinvertebrates. Smaller macroinvertebrates can hide from predators in the leaf litter, while predatory macroinvertebrates can hide from unsuspecting prey in the leaf litter as well. 

Since leaf litter is so important for macroinvertebrate communities, any changes to the amount and type of leaf litter that reaches the stream can impact the macroinvertebrate community. In areas where trees have been cleared along stream banks, the primary energy input is no longer leaf litter. Rather, since sunlight can reach the stream, aquatic algae and vegetation will grow more readily and become the dominant source of energy. As a result, the macroinvertebrate community will shift to a community that primarily eats aquatic vegetation and algae, with consequences for the entire food chain. Similarly, if the composition of riparian vegetation changes and non-native plants become more common than native plants, the macroinvertebrate community may shift as well. 

The best way to maintain and improve this Thanksgiving Feast for our aquatic life is to protect forests along streams, remove invasive species, and plant native trees along streams. Willistown Conservation Trust is working to enhance this feast at Ashbridge Preserve, where we have planted over 1000 trees along Ridley Creek. We hope that these trees will improve water quality and contribute much needed allochthonous input for the inhabitants of Ridley Creek.

By Anna Anna Willig | she/her | Watershed Conservation Associate | As part of our Watershed Protection Program, Anna assists with monthly water chemistry sampling, maintains the tree planting at Ashbridge Preserve, and analyzes water quality data from sampling and from our EnviroDIY sensors.

Flooding 102

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Flooding 102

Last week we looked at the basics of flooding, learning about what constitutes a flood and how floods are measured. This week, we wanted to dive deeper into flooding and explore the causes of floods and how flooding in our area looks different today than it did 20 years ago. If you didn’t get a chance to read our last article, check it out here. 

Where does flooding happen?

When we think about flooding, we are often concerned less by the amount of water moving through a stream and more by the land that becomes inundated with water. The area covered by water during a flood is called a floodplain. Floodplains tend to be flat, low-lying areas adjacent to streams and rivers. Not all floods will cover the entire floodplain, and some floods are large enough to reach areas above the usual floodplain. Since the size of a flood impacts the area that will be flooded, we can expect that a 100-year floodplain, the area that will be inundated during a 100-year flood, to be smaller than a 500-year floodplain. 

How much rain do you need to cause a flood? 

The amount of rain needed to cause a flood depends on many factors. The amount of rain that falls during a storm is not the same as the amount of water that reaches a stream. When water falls in a watershed, it can either be absorbed into the soil, a process called infiltration, or it can flow over the ground into the stream, a process called runoff. In general, more runoff leads to more flooding. 

A big factor that determines the amount of runoff from a rainfall event is the amount of impervious surface in a watershed, the area that feeds into a stream. An impervious surface is any feature that blocks water from infiltrating into the soil, such as buildings, roads, parking lots, driveways, sidewalks. Watersheds with lots of impervious surfaces will have more runoff than watersheds without impervious surfaces. Consequently, more water from a storm will reach the stream in watersheds with more impervious surface cover, causing greater flooding than if there were less impervious surface cover. 

Recent weather conditions also impact the amount of rain needed to cause a flood. Soils can only absorb so much water before they are saturated and cannot absorb any more. If a storm occurs after a few rainy days, when the soils are nearly saturated, less water will infiltrate and more water will run off, causing a larger flood than if the same storm occurred during a dry period. 

The rate of rainfall also impacts flooding. Storms that dump a few inches of rain in an hour will cause more flooding than storms that have the same amount of rain over several hours. Infiltration of rainwater into soil takes time, so more water will be able to infiltrate when a storm occurs over a longer period of time, resulting in less runoff and less flooding.

What is the difference between a flood and a flash flood?

Flash floods are a type of flood that occur quickly, within a few minutes to a few hours of a heavy rainfall. They are marked by a sudden increase in water level and are most common in developed areas with lots of impervious surfaces where there is little infiltration. Flash floods are generally considered the most dangerous type of flood because they can occur with little warning, leaving people little time to evacuate the floodplain.

It seems like we are having 100-year floods every year, why is that? 

In our last post, we discussed that a 100-year-flood was a flood that had a 1% chance of occurring in any given year. However, changes in our watersheds and in our climate are altering flood patterns in our area. 

The amount of impervious surface in a watershed is a major factor in determining how big a flood will be after any given rainstorm. As our area becomes more developed and more impervious surfaces are put in, it takes less and less rain to cause a flood. 

Climate change impacts the frequency and intensity of storms and resulting floods in our area. Researchers predict that a warming planet will cause larger and more frequent hurricanes to form in the Atlantic Ocean. While the brunt of the damage from these storms will be borne by coastal communities, we can expect to see more of these storms reaching our areas, leading to large floods such as those caused by Tropical Storm Isaias in 2020, Tropical Storm Sandy in 2012, Hurricane Irene in 2011, and most recently, Hurricane Ida.  

In addition to seeing the consequences of more severe coastal storms, our area is predicted to see an increase in annual precipitation by 2050. As a result, we can expect to have more intense storms more frequently. The increase in intensity and frequency of these storms, combined with the increase in impervious surfaces in our watersheds, means that what was once a 500-year flood may now be considered a 100-year flood and what was once a 100-year flood may now be a 50-year or a 20-year flood.

What is Willistown Conservation Trust doing to reduce flooding? What can I do?

Protecting land from development is one of the best ways to combat flooding. Land that is undeveloped allows far more infiltration than land that is developed, reducing the amount of runoff that reaches our streams after a rainstorm and the resultant floods. 

In addition, the Watershed Protection Program has planted over 800 trees along a floodplain at Ashbridge Preserve. Planting trees helps increase the amount of water that infiltrates into soil and will eventually reduce flooding downstream.

You can help too! Letting your grass grow a little taller before mowing is a great way to increase the amount of water that your yard can absorb, thereby reducing runoff. Adding a rain garden with native plants to your yard reduces the amount of water that runs off into streams. You could also add a rain barrel to your yard and collect rainwater for later use in your garden or elsewhere. Any action you take that reduces the amount of runoff reaching streams will make a difference in flooding for all those living downstream.

To learn even more about flooding, check out these resources:

Flooding 101

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As we approach the last few weeks of summer heat and humidity, we approach peak seasons for thunderstorms and flooding. Last Thursday, heavy rainfalls led to the first major flood of the year in our area. At Ashbridge Preserve, Ridley Creek rose over 2 meters (7 feet) in about 3 hours, pouring out of its banks. In the wake of these floods, we wanted to take the opportunity to answer some commonly asked questions about the fundamentals of flooding.

What is a flood?

The United States Geological Survey (USGS) defines a flood as “any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream” (USGS, 2019). In other words, a flood happens when a stream breaches its banks, resulting in water flowing over areas that are not normally part of the stream. Floods can occur for a number of reasons, from snow melt to rain to changes in tides. In our area, the most common cause of flooding is rain, often from summer storms, and snow melt after large snowfalls.

Ridley Creek before (left) and during (left) a flood at Ashbridge Preserve in 2018. Photos by author.

How frequently do floods occur? 

Floods can occur several times a year, whenever rain or snowmelt causes a stream to overflow its banks. Small floods, when the stream barely breaches its banks, are more common than large floods when the water pours out of its banks.

The size of a flood is determined by the peak flow of a stream or the greatest amount of water moving through the stream during a flood event. Peak flow can be determined by measuring the height of a stream; it is the highest height during the flood event. Higher peak flows indicate larger floods and lower peak flows indicate smaller floods.

This graph shows water depth over time and nine floods that happened at Ashbridge Preserve in the Summer of 2018. The blue line represents the depth at which the stream completely fills its bank–a flood occurs any time the water rises above this depth. Each spike represents a rainstorm and the peak flow, which determines the size of a flood, is the highest depth in each storm. The star represents the peak flow of the flood that is pictured above.   

Floods are classified by how often we expect them to occur. A 100-year flood is a flood of a given size that has a 1% chance of occurring each year. Similarly, a 500-year flood has a 0.2% chance of occurring each year and a 1000-year flood has a 0.1% chance of occurring each year. However, this does not mean that a 100-year flood will only occur once every 100 years or that a 1000-year flood will occur once every 1000 years. It simply refers to the likelihood of such a flood happening each year. 

How do you stay safe during a flood?

Floods in our area can be dangerous, even life-threatening. According to a recently completed Hazard Risk Assessment by Chester County, flooding is the second highest risk hazard in the county. Floods can damage property, destroy roads and bridges, and threaten human lives. 

Floodwaters are most dangerous for drivers, especially when drivers try to cross flooded roads. Twelve inches of flowing water will move a car, and 2 feet of water will easily sweep a car away. Even just a few inches of water can immobilize a car, stranding drivers in the middle of a road. If you are driving during a rainstorm, do not drive through any floodwater. Floodwater is often dark and murky, making it difficult to judge how much water is actually on the road and if the water is flowing. Turn around and find another route or pull over and wait until the water goes down.

Want to learn more about flooding?

Stay tuned next week for Flooding 102, which takes a deeper dive into how land use decisions impact flooding in our area. Until then, check out some of these resources:

USGS. (2019). Floods and Recurrence Intervals: Overview [Techniques and Methods]. USGS.
https://www.usgs.gov/special-topic/water-science-school/science/floods-and-recurrence-intervals?qt-science_center_objects=0#qt-science_center_objects