Welcome, Anne,
and let's hope we can help you.
Veggies do take a lot of nutrients out of the soil, and all the water you use plus the heat also breaks down the organic material in your soil. So if you're going to plant the same small area every year, you do need to amend the soil each year, and most likely also fertilize to keep it rich enough to support your vegetables, so they will produce.
If you have a ready supply of compost - a lot of counties have composting programs and either sell it cheap or here they give it away free, if you load it yourself - that is the best possible amendment. Also the most cost-effective. Another great amendment I've tried just this year is alfalfa pellets, available at any feed store as horse food. Be sure if you buy this, it has no additives and is 100% alfalfa. It costs about $18 for a 50lb. bag. It works best if you mix it with water and let it absorb for a while, then dig the 'slush' into the whole garden really well. But just spreading the pellets and mixing it in, then watering works too.
Good for you for thinking about the possible runoff into the river! If you use a pelleted, time release fertilizer, it releases slowly and constantly for a good supply to your plants but very little if any excess to drain away. Look on the package for "lasts 4 months" or some other such statement. I use the Osmocote "Smart Release" Flower and vegetable formula for my whole garden. I'm sure other people will chime in with their recommendations, too.
