Sorry to beat this dead horse yet again, but the URL below:
http://geonames.usgs.gov/pls/gnis/web_query.gnis_web_query_form
will get you to a wonderful resource. Select the orthophotos, not the topo
maps. Bits and pieces of linear features such as canals show up beautifully
in the photos available here. Don't be frustrated by the choices under
"Feature Type" -- you may have to experiment a bit. Sometimes the best
pragmatic choice is a nearby "populated place." [3 people and a dog will
usually qualify!].
It is perhaps worthwhile to mention that Roman ruins in Great Britain,
1500-2000 years old, have been found this way (using aerial photos, but not
these, of course) even under fields that have been plowed for centuries.
BTW it was thanks to this list that I learned of this site. Just clip the
URL name to your clipboard and nail it into your browser.
John Ragle