Garden reach is like a swimming pool for boats. It's a place where big boats and ships can go and be safe. Just like how you can't swim in a regular pool unless you know how to swim, boats can't just go anywhere in the water unless they know that it's safe. Sometimes the water can be too shallow or there might be rocks or other things in the way that can hurt the boat.
So, people make special places called garden reaches where the water is deep enough for big boats to go safely. It's like a big hole that's filled with water where boats can come and stay for a while. It's also really important because it's a place where people can work on the boats and fix them if they need to.
It's called a garden reach because it used to be a place where people grew plants and had a garden, but they turned it into a special place for boats instead. So, now instead of growing plants, it's a place for boats to get safe and fixed.