Navigating the Network: Part 2

Trace a contaminant downstream


A contaminate has been released upstream of Dillon Reservoir. As the GIS analyst providing emergency response, you have been tasked to calculate how far the contaminant will travel from the source to an endangered fish population in Dillon Reservoir.


Start by zooming to the contaminant. Right click on the contaminant layer under the group layer called "Network Events" in the TOC and select “Zoom to Layer”.


You can see the point of contamination is upstream from Dillon Reservoir, which contains an endangered fish population.


Next, we will calculate the miles of stream the contaminant will travel before reaching Dillon Reservoir.


To navigate the NHD flow network, first ensure that Utility Network Analyst, an ESRI tool is turned on.


Do this by right-clicking in the gray empty space in the toolbar area. A list of tools will come up. Scroll to the bottom and select the Utility Network Analyst.

The tool automatically selects the available network, HYDRO_NET.


Click on the arrow next to the flag button and you will see there are several tools available.


Here is a description of the available tools: 

 Junction Flag Tool – Creates a starting point for the network trace at a junction (Where flowlines intersect).


 Edge Flag Tool  – Creates a starting point for the network trace anywhere along a flowline feature. Does not have to be at an intersection.


 Junction Barrier Tool  – Creates a barrier and stops the network trace at a junction.


 Edge Barrier Tool  – Creates a barrier and stops the network trace anywhere along a flowline.


Place an edge barrier where the flowline carrying the contaminant, the Blue River, enters the NHDWaterbody containing the endangered fish species, Dillon Reservoir.


Quiz: As you navigate down the Blue River in ArcMap, the feature turns from an NHDFlowline StreamRiver Feature to an NHDFlowline Artificial Path within an NHDArea StreamRiver. Why is this?


See Answer



Learn more about network types.


Zoom to the Contaminant Bookmark. Place an edge flag on the point where the contaminant  is entering the Blue River.



Set the Trace Task to “Trace Downstream” from the dropdown menu.


Click the solve button.


The section of the Blue River between the contaminant and Dillon Reservoir is highlighted in Red.


Go to the Contaminated Blue River Bookmark to see the entire highlighted section.


The red trace results blend in with the HUC boundaries. You can change the color of the trace results or set the trace to select the features identified by the trace.


Next, select the features composing this contaminated segment to calculate the distance the contaminant will travel.


To do this, click the Analysis button on the Utility Network Analyst toolbar and select “Options”.


Under the Results tab, change the Results Format to “Selection”.


Click Ok.


Execute the downstream trace again.


The features are now selected.


Open the NHDFlowline Feature Class attribute table.


25 features have been selected.


Choose the option to only view selected features.  


Drag the scroll bar until you see the LengthKM Field.


Right click on the field and select “Statistics”.



Under Analysis, clear flags, results, and clear your selection.


Tip: Get more accurate results when doing downstream traces by placing barriers at the divergences to force the trace down the main path of the stream.


Tip: The selection will not show up at scales smaller than 1:150k because at this scale you are viewing a copy of the NHDFlowline feature class. This copy is for cartographic purposes and the selection is not based on this layer.


What is the value in being able to calculate the distance a contaminant will travel along the network?


See Answer


Clear your selection.


Clear flags and barriers.


Navigating the Network: Part 3