How to define/delimit/outline a drainage basin. (step 3)
The third step is to add additional tributaries (those not depicted on the topographic map). As you should recall, wherever a river or stream crosses a contour line, it makes a "V" that points uphill. If you examine the map closely, you will see many similar deflections where no rivers or streams are drawn. Starting at the rivers you have already highlighted, you can add additional tributaries (as shown below in light blue).
Now, add some additional tributaries on your own!
Next step: Defining adjacent drainage networks.