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General FAQ's

Barrier Removal Modeling FAQ's

From where does the FPDSS get its barrier/dam information? (top)
The FPDSS receives barrier/dam datasets from various local, state, and federal barrier inventories. After receipt, a dataset is mapped into the FPDSS data structure. Click here for a full list of barrier/dam data sources. Note that barriers are potential barriers and have not been ground truthed by the U.S. FWS.
Why does the FPDSS not have barrier/dam information for my watershed? (top)
The developers of the FPDSS attempt to obtain barrier/dam information for the entire USA. If you have information about a barrier/dam inventory that you would like to see available via the FPDSS, contact us.
How does the modeling operation work? (top)

The FPDSS barrier removal modeling operation allows a user to better understand how the removal of a barrier from a stream affects the local aquatic environment.

The modeling tool allows the user to select one or more barriers in a watershed and calculate the number of stream miles that would be connected if the barrier(s) were removed. The model uses the 1:100,000 National Hydrography Dataset (NHD). The NHD is a database of flow-networked stream "reaches". Walking upstream and downstream of the selected barrier(s), the model adds up all NHD stream reaches, including tributaries, until it encounters another barrier or the end of the stream. The tallied reaches are then added together to provide total stream miles opened.

The information returned by the modeling operation provides only one factor in evaluating which barriers are good candidates for removal. Factors such as the presence of non-native (exotic) species, contaminates, landowner cooperation, and the availability of funds are also very important.

The Modeling Algorithm

Once a user has selected which barriers within a watershed to model, the following algorithm determines which stream reaches are affected by the removal.

Definitions
RemovedBarriers

The set of all barriers to be removed by the algorithm.

AllBarriers

The set of all barriers in the Hydraulic Unit modeled by the algorithm.

OpenedReaches

The set of all NHD stream reaches that are adjacent to a barrier in RemovedBarriers.

BlockedReaches

The set of all NHD stream reaches that are adjacent to a barrier in AllBarriers but not in RemovedBarriers.

Outputs
UpstreamOpenedReaches

The set of all NHD stream reaches opened for upstream fish passage.

DownstreamOpenedReaches

The set of all NHD stream reaches opened for downstream fish passage.

UpAndDownstreamOpenedReaches

The set of all NHD stream reaches opened for up and downstream fish passage.

Algorithm
  1. For each barrier B in RemovedBarriers

    1. Let openedReach equal the reach adjacent to barrier B

    2. Find all NHD stream reaches that are upstream of the openedReach but are downstream of any reach in BlockedReaches. Add this set to UpstreamOpenedReaches

    3. Find all NHD stream reaches that are downstream of the openedReach adjacent to B but are upstream of any reach in BlockedReaches. Add this set to DownstreamOpenedReaches.

      If a confluence is found downstream of openedReach, add each reach upstream of the confluence point, but downstream of any reach in BlockedReaches to the DownstreamOpenedReaches

    end for each

  2. Add any reach in both UpstreamOpenedReaches and DownstreamOpenedReaches to UpAndDownstreamOpenedReaches

What are the limitations of the algorithm? (top)

Spatial Limitations

Two spatial limitations of the modeling algorithm arise from the fact that the base unit of the modeling operation is a single National Hydrography Dataset (NHD) stream reach. The algorithm understands reaches as existing in two states:
  1. Blocked for species passage by the presence of an adjacent barrier.
  2. Opened for species passage by the absence of any adjacent barriers or by virtue of any barrier adjacent to the reach selected as a candidate for removal.
The first limitation occurs when a stream reach is associated with more than one barrier. Since the modeling algorithm doesn't know which of two barriers on a reach is upstream of the other, the model ignores the presence of a second barrier when determining the state of the reach. In other words, if a stream reach is adjacent to a barrier selected as a candidate for removal, that reach is considered opened by the algorithm regardless of the presence of another adjacent barrier. This can result in the model "flowing through" a barrier that is not a candidate for removal and thus giving too high a figure for the number of stream miles opened for passage.

Second, because the algorithm does not know the absolute location of a barrier on a stream reach, the algorithm does not include partial stream reaches in the calculation of stream miles open for passage. Users of the modeling algorithm may note that the color-coded stream reaches signifying reaches opened for species passage may continue past a barrier if that barrier is not at the end of the stream reach. This situation will also give a slightly higher result for stream miles opened for passage.

Why do I get the error "Removal Modeling not Possible"? (top)
If a barrier is not adjacent to a National Hydrography Dataset (NHD) stream reach, the FPDSS cannot determine which reach will be opened for species passage when the barrier is removed. As the NHD becomes more comprehensive, this error will become less frequent.