Floodplain Connectivity

Floodplains are the transitional habitats between a river and the uplands bordering rivers and streams.  The river and its floodplain are linked so that one supports the other and when properly functioning, provide the greatest plant and animal diversity and ecosystem resiliency.  Rivers with connected floodplains are wetter, and in the dry western climates of southeastern Washington aid in the natural production and maintenance of riparian forests.  Riparian forests cool the landscape and create and maintain channel and habitat complexity, creating a feedback loop that once broken requires management and support to restore.  Wetter riparian forests provide benefits beyond fish in that they are less likely to suffer catastrophic forest fires and are more resistant to damaging erosion during major flood events.  The self-supporting nature of connected, properly functioning floodplains is what provides our salmonid resources resiliency in surviving natural and human-caused disasters.

A large proportion of the Tucannon River has been disconnected from its floodplain through land conversion from forested riparian to a more “domestic” landscape.  The river has been shortened and steepened (increasing stream power) and significant effort to maintain it in a static condition has led to loss of natural functioning conditions.   No longer able to disperse the energy of even modest floods, erosive forces damage property, washing away top soil and damaging farm lands and riparian areas alike.  In an effort to adjust to a river that has more ability to damage property, rip rap and river levees have been built but where resources were limited, property has been damaged and lost.  This process has led to a conversion from wet riparian forest to dryer, shrub step landscape, more prone to fire and further erosional forces and the resulting poor habitat and declining fish populations.

Increased stream power has made the Tucannon River channel wider, with higher banks, increasing the size of floods required to wet the floodplain disabling the rivers ability to support riparian or recover from this condition (see Tucannon Floodplain Model below).  The images above present an example from the Tucannon where floodplain was reconnected by removing portions of a river levee to reduce stream power and improve floodplain connectivity.  The upper images shows a pre (left) and post (right) project levee removal and log jam project implemented on private property for the purpose of improving flow onto the floodplain and support the development of channel and habitat complexity.   The left image illustrates a simple channel shape with a straight flow path (blue arrow) and no channel complexity or habitat.  The channel flows along a rip rap armored levee indicated by the yellow arrow, and results in high stream power during high flow that rpoduces little salmon habitat and preventing floodplain connectivity.  In 2015, a project was implemented to connect the left bank floodplain by removing a large section of the river levee and placing log jams (upper right).  The yellow arrow indicates the same perspective along the levee face and the red arrow indicated the point where a log jam was placed to split stream flows onto the floodplain.   The result over the three years since construction has been the connection of floodplain, an increase in gravel storage, raising of the main channel bed and increased connectivity on both the right and left river banks.  The blue arrows in the right image indicate the complexity in flow paths that has developed.

To better illustrate the natural and working landscape of Tucannon River floodplain, the Programmatic has utilized a floodplain model (above) that demonstrates the changes from connected floodplain in natural condition in comparison to a number of floodplain scenarios ranging from disconnected to restored.   Due to the amount of salmon habitat (floodplain) being located on private working farms in the Tucannon, two separate models were developed to illustrate floodplain connectivity for both floodable and non-floodable working lands.  The model illustrates the effect to channel complexity and increased water table supported by floodplain connectivity.

Connected floodplains recharge shallow groundwater and inundated landscapes add nutrients and remove fine sediment and contaminates harmful to juvenile fish.  Water reentering the river from floodplain storage maintains higher summer flows and reduces summer stream temperatures.  The three images above are taken on the Tucannon floodplain in April 2019, in areas which were not typically flooded prior to restoration actions.  The upper left is a long high flow channel forming in the floodplain at ~450 cfs following a 1,100 peek flow in 2019.  Channels like these help reduce flood damage to salmonid redds and juvenile fish in the river by reducing the rivers energy and ability to do damage.  These channels also provide refuge and available food to juvenile fish, increasing river productivity.  The two photos to the right are locations where the floodplain has become inundated all year providing fish access out of channel habitat year round.  This benefits fish because floodplains provide more abundant and diverse food resources to juvenile Chinook and steelhead helping them to grow faster than main channel siblings.  A study completed by researchers in California’s Sacramento Valley found spring Chinook raised on the floodplain grew > 2x faster than those raised in the river disconnected from the floodplain (Katz et. al. 2017), illustrated in the image to the left.

Tucannon Floodplain Model

Tucannon Connectivity Benefits