Here is a general description of a way to approach the bankfull-width problem at the end of the modelbuilder tutorial. There are other ways to do it, and if you have a different plan, give it a try. But here is a general approach you can follow.
1. Extract by Value the cells in the flow accumulation raster with values > 100 or 200 (some large value that gets you only the streamline locations – probably worth using the same value you used elsewhere in the model – you can feed that in as a variable)
2. You could do math now or later, so might as well do it now – using raster calculator, multiply the values of your extracted raster so that they represent square kilometers instead of number of cells – so `cell value * (cellsize^2)` or most likely something along the lines of `VALUE * 30 * 30`. You could also do that later in the field calculation
3. Convert the raster to points so each cell is now a point
4. Spatial join the points to your streamlines with an appropriate search distance. Make sure to select JOIN_ONE_TO_ONE and an appropriate merge rule (something like first, or max) for the drainage area value field
5. Then create a field on your streamlines that will hold your bankfull-width distance and use the Calculate Field tool along with the formula in the lab and your drainage area attribute that is attached after the spatial join to calculate the bankfull-width attribute (edited)
6. Then use the Buffer tool, with that newly calculated field as your buffer distance
There are other ways to do it, but that’s one way to approach it in ArcGIS.