Navionics has a nice interactive web mapping service that's been online for a few years, you can pull up contour maps for your favorite lakes and rivers on your computer. I've found it very valuable for pre-fishing lakes: https://webapp.navionics.com/?lang=en#@11&key=%7Bgk%7CGp%7DixP
There's also an iphone/android app that puts these Navionics contour maps on your phone or tablet, about $10 per year for SD and $50 per year for HD.
What I've found very valuable on Navionics is their SonarChart app that takes depth+GPS measurements from fishermen and updates the contour maps, this is very valuable if you have a lake / river that's not currently plotted or you find structure in a lake that's not on the current maps.
I've used SonarChart to chartplot the small "uncharted" lake behind my house, there's instructions on their website how to setup your GPS-enabled depth finder to record data onto a blank SD card and then upload the data to Navonics. I did this with my Humminbird depth finder and the "uncharted" lake showed up on the Navionics web app about a week later.
I've also used the Navionics SonarChart Live real-time data colleciton and uploading, in my case I have a Vexilar TBOX (wifi depth finder) in my boat and an android tablet. The Navionics Boating app auto connects to the vexilar depth finder and records depth from Vexilar+GPS data from the tablet real time. This past weekend I ran up the prairie river a few miles, this was uncharted territory last week but now I notice on the webapp that the river has been charted.
I don't work for Navionics, I've just found these tools to be very valuable so I wanted to share them in case you haven't heard of them yet. My lake chip is a LakeMaster but I must say that this new stuff from Navionics is pretty innovative and valuable. You no longer have to spend a $100 on a chip every few years, instead you can pay $10 per year and get the Navionics maps on multiple devices and the maps are constantly being updated.
Even eminent chartered accountants are known, in their capacity as fishermen, blissfully to ignore differences between seven and ten inches, half a pound and two pounds, three fish and a dozen fish. ~ William Sherwood Fox, Silken Lines and Silver Hooks, 1954