Create watersheds to calculate upstream area

Atlantic salmon are an important aquaculture export for Canada. In the coastal province of Nova Scotia, many dams have been retrofitted with artificial waterways so Atlantic salmon can swim upstream and access fresh water, ensuring that the salmon can reproduce. One of the dams in the Mersey River Hydro system needs maintenance work, which will cause the dam's fishway to temporarily be inaccessible. You need to calculate the area of the watershed that will be temporarily interrupted to assess the financial impact on the Atlantic salmon population.

This tutorial was last tested on June 9, 2026.

View final result

Requirements

• ArcGIS organizational account with a Publisher, Facilitator, or Administrator role (see options for software access)

Create watersheds to calculate upstream area

Navigate to the area of interest

First, you'll find the Cowie Falls power plant, where a fishway will temporarily be unavailable, and use satellite imagery to examine it. Then, you'll save the map to your account.

1. Sign in to your ArcGIS organizational account.

   Note:

   If you don't have an organizational account, see options for software access.

2. On the ribbon, click the Map tab.

   

   A new map opens. First, you'll locate the Mersey River in the Canadian province of Nova Scotia. Multiple hydroelectric dams have been built along the river as it flows southeast to the Atlantic Ocean to generate electricity. For this analysis, you'll focus on the Cowie Falls hydroelectric dam, which will temporarily be taken offline for maintenance.

3. On the map toolbar, click the Search button.

   

4. In the Search bar, type (or copy and paste) -64.770857, 44.073927 and press Enter.

   The map zooms to the Cowie Falls hydroelectric dam.

   

   To get a better understanding of the dam and its surroundings, you'll switch to an Imagery basemap.

5. Close the Search result pop-up.
6. On the Contents (dark) toolbar, click the Basemap button.

   

   The Basemap pane appears.

7. In the Basemap pane, click Imagery Hybrid.

   

   The map changes to show satellite imagery of the dam. On this basemap, to the north of the dam, there is a small ladder-like structure bypassing the dam. This channel acts as a fishway, allowing fish to bypass the obstruction as they travel up and downstream.

   

Set the unit of measurement

Next, you'll save the map and ensure it uses the proper unit of measurement for your analysis.

1. On the Contents toolbar, click Save and open and choose Save as.

   

2. In the Save map window, enter the following information:
   • For Title, type (or copy and paste) Cowie Falls fishway disruption analysis.
   • For Summary, type (or copy and paste) This map analyzes the upstream watershed area that will be temporarily unavailable due to maintenance on the Cowie Falls powerhouse.
3. Click Save.

   The map is saved to your account. Before running the analysis, you'll ensure that your map is using metric units. Because your goal is to calculate an area, you'll want to use the metric unit square kilometers.

4. On the ribbon, click your account name and choose My settings.
5. On the My settings page, under General, scroll down to Units. If necessary, choose Metric.

   

   Your account is now set to use metric units of distance and measurement. You'll reopen your map.

6. On the ribbon, click Content.

   

7. On the My content page, for your Cowie Falls fishway disruption analysis map, click the options button and choose Open in Map Viewer.

   

Quantify inaccessible habitat

As a result of necessary maintenance on the hydroelectric dam, much of the Mersey River watershed will be inaccessible to Atlantic salmon. You'll use the Create watersheds tool to calculate how much of the upstream watershed will be affected. First, you'll change the basemap again to one that shows more detailed hydrologic data.

1. On the Contents toolbar, click Basemap. Scroll down and click Living Atlas.

   

2. In the Add layer pane, in the search bar, type Environment Map. In the list of results, for Environment Map, click Add.

   

   The map changes to show the Environment Map basemap. This map emphasizes the rivers and streams in the region. It's also currently configured to show water bodies and labels as a reference layer, meaning that it will be superimposed on your other data.

3. In the Add layer pane, click the Back button.

   

4. In the Basemap pane, for Current Basemap, click Environment.

   

5. In the Reference group, drag the Reposition: Environment Surface Water and Label button to the top of the Base group.

   

   The Environment Surface Water and Label data is now in the Base group and will draw under other layers you add to the map. For now, your map doesn't visibly change because you don't have any other data on it.

   

   Now that the basemap is set up, you'll begin the analysis.

6. On the Settings toolbar, click the Analysis button.

   

7. In the Analysis pane, click Tools.

   

8. In the Tools pane, expand Find locations and click Create Watersheds.

   

   The Create Watersheds tool calculates the drainage area of a point based on ArcGIS Online hydrologic data, curated and authoritative data maintained and hosted by Esri. The first parameter that the tool needs is Input point features, the origin point or points for which the upstream area will be calculated. Depending on the data, you can use existing points from a feature layer, or you can draw your own input feature.

   Tip:

   To learn more, click the information buttons next to the Create Watersheds tool and its parameters.

9. In the Create Watersheds pane, for Input point features, click the Draw input features button.

   

10. On the map, click in the middle of the river above the dam as close to the middle of the channel as possible.

    

    When placing this point, you should consider how close to put it to the drainage line, or middle of the river channel. In this case, it's difficult to see where the main drainage line is because the head of the dam has formed a lake, and below the dam there are two channels flowing downstream. While you can use the Search distance parameter to snap the point to the nearest drainage line, you still want to ensure it's as close as possible to make sure the correct watershed is delineated. Placing this point in the middle of the two downstream channels is a good choice because it can be snapped to either.

    Next, you'll choose the search distance. The search distance helps snap the point to the highest water flow accumulation location to delineate the right watershed. This location depends on the stream network and the resolution of the preprocessed hydrologic data used by the Create Watersheds tool. In selecting a search distance, you should consider several factors. From the tool's documentation, you know that the resolution of the hydrologic data used in this tool has a resolution of 90 meters. The width of the dam itself is longer than 90 meters.

11. In the Pour point settings group, for Search distance, type 90. Ensure the Distance units parameter is set to Meters.

    

12. For Result layer, type Cowie Falls upstream and add your name or initials.

    Note:

    You cannot create two layers in an ArcGIS organization with the same name. Adding your initials to a layer name ensures that other people in your organization can also complete this tutorial. Once a layer has been created, you can rename it in the map to remove your initials, which will not affect the name of the underlying data layer.

13. Click Run.

    When the tool finishes running, two new layers are added to the map as a group layer: the watershed layer and an adjusted points layer. The adjusted points layer contains the actual location used to calculate the watershed.

Explore the results

Next, you'll review the results of the analysis. You'll to check where the adjusted point was placed and ensure it was placed close enough to the main channel that it snapped to the correct watershed.

1. On the map, zoom out until you can see the entire Cowie Falls upstream watershed polygon.

   

   Using the basemap, you can ensure that the watershed area calculated is roughly the same as the watershed delineated on the basemap. There may be small variations in some areas depending on the resolution of the data used in the analysis.

2. Compare the watershed boundaries on the basemap with the Cowie Falls upstream watershed polygon to ensure the pour point used was correctly placed.

   Once you've made sure that the correct watershed was calculated, examine the results that were added to the Layers pane.

3. On the Contents toolbar, click Layers.

   

4. In the Layers pane, expand the Cowie Falls upstream group. For the WatershedFeatures layer, click Options and choose Show table.

   

   In the table, the Description column shows what data was used to generate the watershed area. Because the area of interest is in Canada, the HydroSHEDS 90m raster product was used. You can use this field to check the search distance parameter you chose—it shouldn't be less than the resolution.

5. In the table, find the Area Square Kilometers field.

   The affected upstream area is around 1,889 square kilometers.

   Note:

   Your result may be slightly different depending where you placed the adjusted point.

   Next, you'll check the adjusted point to see how far it is from the sketched point. The Adjusted Points layer shows the actual location used to generate the watershed area after the input point was snapped to the closest drainage line.

6. Close the WatershedFeatures table. In the Layers pane, for the WatershedFeatures layer, click the Hide button to hide the layer from the map.

   

7. For the Adjusted Points layer, click Options and choose Zoom to layer.

   

   The map zooms to the Adjusted Points layer.

8. Zoom out until you can see both the point you placed on the map and the Adjusted Points layer.

   

   The two points aren't far apart. The adjusted point was clearly placed on what is considered the main flow path of this river. Now that you've verified the results, your analysis is complete.

9. On the Contents toolbar, click Save and open and choose Save.

In this tutorial, you quantified the area of the watershed upstream of the Cowie Falls dam that will be temporarily inaccessible during maintenance operations. The results of your analysis will help you assess the habitat area that will be unavailable to salmon and other aquatic species during this time to understand the potential financial impact due to reduced salmon populations.

You can find more tutorials in the tutorial gallery.

Acknowledgements

• Nova Scotia hydrology data was acquired from GeoNOVA.
• World Imagery layer sources: Esri, Vantor, Earthstar Geographics, and the GIS User Community
• Environment Map sources: Esri, TomTom, Garmin, FAO, NOAA, USGS, OpenStreetMap contributors, World Wildlife Fund, Inc., and the GIS User Community