Calculating distance of river route

[MartinLively]

GIS newbie here, so my apologies in advance if I get terminology wrong.

I'm using ArcGIS Pro to calculate the distance of an upcoming canoe trip and create a route map onto which I can later add data collected while I float the route. I'll be traveling parts of West Fork Big Cabin Creek and Big Cabin Creek (but not the full length of either).

I've created two new feature layers. Each layer contains the full length of one creek.

 

I'm struggling with the following items. Any help on any item is much appreciated. 

• Both layers are marked as non-editable. I don't know if that's relevant to answering the questions below.
• I need to trim off the northern parts of both creeks that are not part of the route.
  • I think I need to do this separately for each creek since they are separate layers.
• After trimming the excess and leaving only the route to be traveled, how do I calculate the length of my route? 
  • I'm OK doing this separately for each creek and adding the totals.
  • I know I could view the attribute tables and add up the lengths of the individual segments, but I want to automate this step.
  • Do I need to merge all the segments into one long segment for each creek/layer? 

Again, I'm new to GIS, so don't be surprised if I ask for clarification. Thanks in advance!

[DanPatterson]

Depends on whether the streams are polygon or polyline layers.  (rightclick on the layer, open the table or check its properties)

If they are polylines and it is in a projected coordinate system, then the length should be given in meters (or feet).  If not, "export" the layers to new layers, specifying an appropriate projected coordinate system (eg UTM, state plane) and the lengths of the streams will be calculated for you in each table.

Enough for now... because you can do things like split the polylines etc at the reaches and other things.

[MartinLively]

Thanks, Dan. This is helpful. The streams are polylines in a projected coordinate system. As you said, the attribute table lists the length in km of each stream segment.

What tool can calculate the combined length of several individual stream segments (or even the whole stream)? For context, one of these streams has 44 stream segments with individual length values. 

[DanPatterson]

Summary Statistics (Analysis)—ArcGIS Pro | Documentation

with a "case" field if there is one to identify stream segments if they are differentiated in the table.

Otherwise, the "sum" will give you total of all segments if a "case" can't be identified