arcgis-desktopgeneralizationline
I am looking for a way to connect line segments that are unconnected. It is a shapefile for a network of rivers and all the lines are slightly disconnected.
Is there something like aggregate polygons but for lines?
Edit
My river data is not continuous but I would like it to be so I can run simplification tools on it for a generalized map.
The lines don't always touch so when I try to generalize, it doesn't work. I want to connect the line together automatically.
I've devised a way of doing it with little bits from all of the other answers.
The scripts that @GeoJohn and @FelixIP have created may well work (if you have access to ArcGIS 10.1 and the database access functions), so if someone else had this problem please check those options out as well.
I did the following in the end:
Created a very small buffer polygon around the lines (0.1 metres) to make sure there was crossover between the lines that were connected.
Use the dissolve tool with the Unsplit option checked. This created individual polygons for each of the connected line groups.
Create a unique ID for each polygon.
Use the spatial join tool to add the unique polygon ids to each line that intersects or is within the polygons.
Use the dissolve tool again to dissolve the lines based on the unique ID.
That has worked for me but like I said the scripting methods may be preferable for other people.
The problem can be solved in several simple steps - 5 in total as you can see below. The core of the solution conists in using the Join attributes by nearest tool.
Let's say you have a the following line segments with white dots representing start- and end points and the number the id of the line segments. Some segments are connected (end point of 1 = start point of 2), other not, there are gaps in between, like between segment 2 and 3. In the following screenshots, you have 19 line segments that can be grouped to 8 groups of connected lines as follows:
1,23,4,5678,9,10,11,121314,15,16,17,1819You see this here on the screenshots: there are totally 8 gaps:
To join the segmentes that are connected to one single line, proceed as follows:
Select Menu Processing / Toolbox / Join attributes by nearest, set both input layers to your line layer and Maximum nearest neighbors to 1. You could also define a Maximum distance (to get your extra). Run the tool.distance. This tells you the distance between neighboring segments: if it is 0, the two segments are connected. Those lines that have a value > 0 are no. 6, 7, 13, 19: exactly those we identified above as not being part of a group of connected line segments. To get your extra condition, you could define a distance threshold: if the distance is smaller, you could consider the lines as "connected".
select by expression symbol in the toolbar (see screenshot above) and paste the expression "distance" = 0 to select all segments that have a distance of 0 to their nearest neighbors - thus segments that are connected.Merge Selected Features symbol in the toolbar: 
Menu Vector / Geometry Tools / Multipart to singleparts. At the same time, this also solves our last problem: the one large line gets split into several lines that are connected: Now, we have exactly the merged lines that we defined at the beginning. You can see this in the next screenshot - red dots are start- and end points of the new line, the number of the line labels represent the totally 8 new features (lines). As you see, the do not follow the original order any more, but that should not be a problem.
Best Answer
You can create a Geometric Network based on your hydro feature class, as described in this Esri tutorial. If you want to go a quicker route, use the Create Geometric Network tool - for this you will need to have created a spatially referenced Feature Dataset (stored in a geodatabase of some kind), within which you will save the Network.
When you go through the options to create the Network, you can set the "Snap Tolerance" to a distance value specifying the max distance where gaps will be snapped together. Obviously, the input feature class will be your hydro features.
When it is created, you can re-create with different snapping tolerance using Rebuild Geometric Network if necessary, but you can either convert back to a new feature class or else simply display the Geometric Network on your map, with no added generalization necessary.