Searching for downstream sites

• RivEX will generate an Excel file listing all downstream sites for any site within a single layer.  Sites that have no sites downstream of themselves are coded with a -1.  Therefore all sites are returned in the output as either -1 or a list of downstream IDs with distance from mouth.

• To use this tool you must complete the parameters section of the main interface and press the Build button.

• Select the Find Downstream Sites tab and complete the parameter box as shown in the example below. Sites must be snapped to the network and the ID must be a numeric value.

• You will asked to confirm for each point layer if the points are snapped to the network. Go here to find out about using RivEX for snapping points to the network.

• Select the catchment ID and distance from river mouth fields.  These are values encoded into the network by RivEX.  Ensure your network has these encoded before you attempt to use the tool.

• Click on Go! to start the processing.  RivEX will first check if the sites have unique IDs and intersect the polylines of the network. If any are found not to be intersecting it will drop these from the analysis and report the number dropped in a message box.

• Once the analysis has completed RivEX will display a message box informing you that it is about to transfer the data to MS Excel. Click on the OK button but do not interact with the keyboard or mouse during the data transfer.  Excel will minimise when completed.

• Typical output is shown below.  A -1 indicates that the site has no downstream sites. The count of site ID tells you how many downstream sites there were. The downstream site ID gives you the order they were visited, but use caution if your downstream sites were distributed on multiple branches of a braiding river system (see below for more details).

• If the number of downstream site combinations exceed 65,536 then RivEX will direct output to a dBase file to avoid exceeding the capacity of MS ExcelXP/2003.

• In the scenario where you have downstream sites of either side of a braid (2 & 4 in the below image) RivEX does not treat them as mutually exclusive.  RivEX does not know which side of the braid you want to travel down and there is usually no coding within the network that defines main channel.  If your river network has bifurcations within bifurcations then the combinations of routes from site to river mouth can be large.  Currently the only way to resolve this is to simplify your downstream site data so that you only ever consider a single point on any branch of a braiding or anastomosing channel.  Imagine you are studying fish migration and you have weirs on both branches of a bifurcation, this would be important to know but RivEX would report them as if they were sequential.

• A traverse downstream will pick up sites on either side of any braid, thus in the example above the sequence reported by RivEX for site 1 would be 2, 4 & 3. The sequence is dictated by the sites distance from river mouth. If 2 and 4 happened to be exactly the same distance from the river mouth then the reporting sequence will be dependent upon the order the polylines were digitised.

• If you have downstream sites in exactly the same location as you input sites then these will be counted as downstream.