Alluvium soil is a deposit of clay, silt, sand, and gravel left by flowing streams in a river valley or delta, typically producing fertile soil.
This is some of the most fertile soil on earth. In fact, over 12,000 years ago, our agricultural beginnings nearly all started around creeks and rivers. (Carrying water is hard work).
Rain, snow, and flowing water erode rocks, carrying minerals downhill, where they collect in lowlands with regular supplies of water. Nutrient rich alluvial soil plus readily available water and you have it - a garden!
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.