Hey, Stan. I would not be surprised if the rocks in the basin were not absolutely clean when they were put in or that there are leaves, insects or other items in there. In any case, some dirt would come from the air or any rain you might have had. If there was any splashing outside of the basin there would be mud. Any debris at all can cause the growth of anerobic bacteria which is the cause of the sulphur smell. That kind of bacteria thrives when there is a lack of oxygen in the water and that is what happens when the pump is turned off. It takes only a few minutes for the good bacteria to begin to die off.
Since we don't know how the waterfall and basin were constructed it is hard to give any detailed advice. I am going from what I learned when I designed one almost seven years ago. The best I can do is to say the water needs to be changed out, the basin cleaned and kept clean with a filter that you can access easily for regular maintenance and the pump needs to run 24/7. Maybe there is someone else on the forum who can give you better advice. Sandy

I too have a pondless waterfall in my backyard. I go several days at a time without running it. When I do, the smell is just horrible. It's real bad for about 10 minutes then it begins to subside. I add a little chlorine and that helps a little with the water clarity but does little for the smell. I really don't want to run it all the time but I don't know what else to do to at least minimize the odor. I change out the water about once a month. I suppose I could do that more often but it's a pain to do. I was hoping there was some type of product out there I could add to the water that would help. If anyone knows of a product that would help, please let me know. Thanks... Mike

You could try to add Activated Carbon to the water to adsorb odors but so long as you are not running the pump and moving the water you are reducing the oxygen content and encouraging the growth of anaerobic bacteria which is what causes the odor in the first place.

Either replace the water on a daily basis or set up a decent filtration system with moving water and use it or the odor will continue. What you have now is pretty much a producer of bad bacteria.

Anyone who claims you can turn off the pump for long periods really does not know what they are talking about.

I totally agree with Sleepless, the water needs to keep moving or it becomes an stagnant mess. Is there a possibility that you can drain it when not in use and fill it when you plan to use it?