If your HDS 7 has a lake mapping package loaded into it then all lakes in the package will display. The cartography is all one software package. Your HDS unit will be a "Lake Insight" or "Insight USA" unit. You can check other lakes by using the arrow keys and moving the cursor over Lake Shasta for example an look for the contour lines and detail mapping.
The easiest way to do this is to go the the Chart page, zoom out, use the arrow key to move the cursor over Lake Shasta or another lake, zoom in to view the mapping.
You can get a Lowrance Insight mapping card to add mapping to a Base Mapping unit. The Base Mapping units only have vague mapping in them and don't have any contour lines and will need a mapping card inserted into them. If you get a card, go to the menu and select the card in the map data so the unit will read the card and display the mapping you want.