Definition: Stream fragmentation occurs when roads, culverts and dams impede fish from moving up and downstream of the structure and cause an interruption the natural connections within a stream network. Barriers to fish movement that separate streams into fragmented sections result in what is often called "reduced stream connectivity."
Actual Ecosystem Stress: For fish and other aquatic organisms that depend on being able to access different habitats for different stage of their life cycle or to survive different seasons, the effects can be detrimental. As habitat becomes fragmented, fish populations can decline or even disappear. Limiting interactions among nearby populations can reduce genetic diversity over time. For aquatic organisms like freshwater mussels that depend on host fish for distributing their larvae upstream, this lack of connectivity can eliminate populations.
Sources: Perhaps the best known example of large scale stream fragmentation is found in the Northwest, where dams have blocked migrations of salmon between freshwater spawning habitats and ocean feeding habitats. Without access to both, salmon cannot survive. Stream fragmentation can also result from less obvious and much smaller scale activities. Some of the most widespread contributors to stream fragmentation are roads. In addition to the sediment that roads often contribute to our rivers and streams, road construction requires culverts or bridges to be installed at stream crossings. Culverts generally increase water velocities and are often not passable for fish and other aquatic critters, especially during very high and very low flow conditions. Other sources of stream fragmentation include dams, small irrigation diversion structures and canals, dewatering, and even water quality and habitat degradation that create uninhabitable conditions along critical migration and dispersal corridors. Without access between required habitats migratory fish populations ultimately perish. Without refuge habitats and some level of immigration and dispersal (i.e. new genes), resident populations become much more vulnerable to extinction.
Mitigation of Threat: Reconnecting fragmented habitats has become a major focus of watershed restoration efforts, and can be accomplished in many ways. Impassable road culverts can be fitted with baffles to allow fish passage (figure 1) or replaced with bridges (figure 2). Fish ladders can be installed at small irrigation barriers to allow upstream passage (figure 3) and screens can be installed at diversion headgates to prevent migrants from being trapped in irrigation canals (figure 4). Habitat and water quality degradation can usually be repaired using some combination of restoring riparian vegetation, stabilizing streambanks, and fencing to exclude livestock. In some cases, active channel restoration might be required. Sections of stream that were channelized to increase farmland or reduce flooding can be placed back into historic meander bends, and trees can be felled into stream channels where natural recruitment of large woody debris was reduced by historic logging practices. As with any restoration effort, the first step to reconnecting fragmented stream habitats is to identify the root cause of the problem. Once that has been accomplished, the project can move forward with a proven technique to address that problem.
|Figure 1: Baffles placed inside of culverts slow water velocities and allow fish and aquatic organisms to move upstream. Left Credit: Bureau of Reclamation. Right Credit: NY State Dept. of Environmental Conservation.|
|Figure 2: This undersized road culvert which had blocked upstream fish passage during high flows and caused sediment aggradation (left) was replaced with this bottomless arched culvert. Credit: US Forest Service|
|Figure 3: This fish ladder was constructed to allow fish to swim upstream around a 13-foot high diversion dam on Rattlesnake Creek, near Missoula, Montana. Credit: Bureau of Reclamation|
|Figure 4: This fish screen was installed in an irrigation canal on the Thomas Fork of the Bear River in southeastern Idaho to prevent native Bonneville cutthroat trout from being trapped in the canal. Irrigation water enters the structure and passes through the angled screens on either side, then continues down the canal to irrigators. Fish and debris are routed back to the stream via a bypass channel (right background). Credit: Warren Colyer|
Dams that no longer serve their intended purpose or cause significant ecological damage can be breached or removed. Because dams slow down water and expose it to air temperatures and sun, they often warm water temperatures and their stagnant waters can encourage algal growth which can reduce dissolved oxygen. In addition, dams often bury valuable spawning habitat. Dam removal (Figure 5) is gaining popularity as a method of restoring large sections habitat in rivers and streams as well as improving water quality.
Figure 5: Excavator beginning removal of the Cuddebackville Dam on the Neversink, 2004. Credit: George Schuler TNC
USFS Fish Friendly Stream Crossings
Exploring Dam Removal
National Fish Passage Program
Fish Passage Design Guides
Dam Removal Success Stories
Oregon Stream Crossing Guide
Penobscot River Restoration Trust
Research on Fragmentation&Genetic Effects