All rowing movements are very effective upper-back mass builders – for both width and thickness. But as you have found out, these movements also place a lot of stress on your lower back. A lot of lower-back injuries have occurred from sloppy bent-over rows and cable rows.
One of the best exercises I have found to get the same stimulation without the strain to your lower back are “V” Bar Pull-Downs and close grip pull-downs. You can almost completely eliminate any lower-back stress by following this technique – as you pull down toward your chest, it’s important to lean back approximately 45 degrees. This is a row movement, but the pulley is overhead, so it removes the strain on your lower back. And as you lean back as you row toward your chest, you target the back muscles just as you would with conventional rows.
There are other exercises you can do as well if you have the right equipment. At our gym here we have a seated, plate load row machine that allows you to do a row while eliminating your lower back from the movement. It’s a nice machine, but it’s still a machine. I’m not fond of machines because they “structure” the exercise movement. This “structure” is rarely biomechanically correct. However, when you are injured, you play under a different set of rules. Machines can then become quite handy, therapeutic, and effective.