You appear to be getting this from the Time-Life Mysteries chunk of the Internet, where I'm seeing lots of gushing about ancient irrigation systems in the Kalahari. (Representative example here.)

Go a little deeper than Google Maps, here- you're looking at one low-resolution photograph. The problem is magnified in Google Earth, because these features are smoothed out by algorithms meant to create 3D images of human streets and buildings- they're going to suggest a world full of more polygons and straight lines.

What you're looking at is the Kalahari Desert, and the Kalahari is full of - everyone say it together - SAND. Sand, when blown by consistent winds from one direction, forms dunes. These dunes take on a rhythmic, linear appearance. I could go on, but I'm going to go straight to a source which also relies on aerial photography - this page from NASA's website.

...NASA's Terra satellite captured this image on May 7, 2007... the long, thin diagonal stripes are linear dunes.

So what you're seeing are not shadows within canals, but shadows cast by tall dunes.

Fascinatingly enough, you have accidentally stumbled onto a mystery; the dunes didn't make sense given wind and vegetation patterns in the Kalahari. According to NASA, researchers in the 1980s and 1990s drilled into the dunes and found that they likely formed between 28,000 and 10,000 years ago, and enough vegetation now survives to hold their shape largely intact.

So, this is a fun exercise which shows us a few things.

• If a claim invites skepticism, dig a little deeper.
• Look at everything from more than one angle (literally, in this case).
• Google returns what you ask for. If you search for "ancient canals Namibia," you'll get a flood (ha) of information from Blogspot ranters and Papyrus-font researchers and all kinds of woo-adjacent flakery. If you search for "lines Namibia," this is the first image result - and that led me quickly to solve this mystery.
• Observing something through your screen is not observing something. On-the-ground, practical experience is vital. We are, in general, disconnected from the way the world works, and we are addicted to the instant authority of the tools we seem to command. It is necessary to humble ourselves and accept that we need to look harder, think harder, and be better.
• Most importantly: Your curiosity and your passion are to be commended. You're fascinated about what the world contains. You're excited that there could be stories hidden everywhere. You're goddamn right. The fact that the conclusion does not match your hypothesis should not disappoint you. You learned something today, and your question allowed lots of other people to learn something, and that is how science and knowledge are built. That's about the only thing that truly lifts humans up and makes them special. Don't be discouraged. Keep exploring.