Threats - Flood Response

Definition and Background:  Flooding is an expected and normal occurrence in river systems. Floods range in magnitude from small biennial events which crest just above bank height, to large infrequent events that inundate sizable areas of low-lying land in the river corridor. All of these events have beneficial aspects. They scour sediments, replenish floodplain soils, redistribute materials and debris, and help prevent non-native species from establishing themselves in the river corridor. But flood events, especially large ones, can dramatically alter aquatic and riparian habitat - sometimes resulting in negative impacts to indigenous fish species, including brook trout.

Many of these impacts occur naturally. However, they are sometimes the result of human response to the flood. Those of natural origin are usually localized and temporary, and typically associated with the transport of materials from one place to another by the floodwaters. For example, in one location a scour hole may develop behind a mid-channel boulder, while elsewhere a pool may be filled with sediment transported from upstream. Similarly, spawning gravel may be relocated from the head of a riffle to a point bar downstream. Woody debris may become stranded on the floodplain, only to be remobilized by the next larger flood. Channel slope may increase when an oxbow is created due to an avulsion, while elsewhere slope may decrease as bed and bank materials are deposited where the channel widens and stream power is decreased.

Functioning floodplains with intact vegetation can help trap large logs and debris, preventing them from lodging on bridge pilings

All of these changes can negatively impact fish, including brook trout. For example, a flush of sediment may accumulate at the mouth of a tributary stream, thereby making upstream passage to spawning areas and coldwater refuge more difficult. Or, the stranding of woody debris on the floodplain may decrease the availability of instream cover. As a result, brook trout may be forced to relocate to find the resources needed to survive. However, such impacts tend to be short-lived, and are often reversed during the next flood event.

Actual Ecosystem Stress:  The effects of human response to flood events, on the other hand, are often more lasting and detrimental. Well-meaning, but uninformed people sometimes undertake a variety of activities intended to 'repair' the stream. Often, these activities have the opposite effect. Rarely, are they beneficial for brook trout.

Sources:  Human response to flooding may include the removal of snags and debris jams, channel dredging and straightening, bank armoring, gravel mining, and the construction of flood control dams and levees

Gravel removal from the channel is often a typical response to undersized bridges and culverts.

Removing snags and debris reduces the diversity, complexity and abundance of brook trout habitat. It also reduces the availability of litter and detritus for benthic macroinvertebrates, an important food source for brook trout. Gravel mining, channel dredging and straightening usually have the unintended effect of perpetuating channel instability. As a result, brook trout may be forced to search continually for suitable habitat to provide the resources needed for their survival. Bank armoring increases flow velocities and transfers erosive forces from place to place. Higher velocities pose challenges for juvenile fish. Armoring reduces near-bank vegetation and canopy cover, leading to localized increases in water temperature. Depending on their construction, flood control dams trap spawning gravel and other sediments important for the natural lifecycle of brook trout.

Left: Use of heavy machinery has reduced roughness, created a berm and effectively increases flood energies, which may lead to increased erosion downstream and headcut development upstream.
Right: Bank armoring with rip-rap is often expensive, reduces riparian function, and increases water velocity, which often leads to increased bank erosion up and downstream.

Measures to stop or mitigate threat:
See TU's Restoring Streams to Reduce Flood Loss [1]

• Avoid heavy machinery use in streams
• Identify sources of flood damage to infrastructure, such as undersized bridges and culverts
• Identify floodplains that no longer function as flood relief and investigate ways to encourage flooding where appropriate
• Setback existing levees to give river access to more of floodplain to flood
• Breach or remove levees where appropriate to encourage floodplains without infrastructure to flood and reduce water depths and water velocities in main river channel
• Increase riparian vegetation where possible to increase bank strength
• Avoid gravel removal and subsequent channel instability and erosion
• Investigate possibility of floodproofing structures or removing infrastructure in chronically flooded areas
   

Left: Floodplain drain culverts to the right of the bridge allow flood waters to pass under the road bed, reducing the "backwater effect" and subsequent aggradation upstream of the bridge. Credit: Delaware County Dept. of Public Works, NY.
Right: Breach in old railroad bed allows river to access floodplain and reduce flood water depths and flood water velocities.

Further Reading:
Luna Leopold 2000 publication on alluvial rivers [2]
USFWS River Restoration [3]
Technical article on dams, gravel mining and river channels [4]