Once a friend of mine, who is a history-geography teacher just like me, told me there was a competition in their school. One of the questions was: How many Danubian islands are there in Budapest? He told me what he tought the correct answer would be, which was an irrationally big number, and asked me if it is true. I told him: it depends. What do you call an island?
In my opinion, island is a peace of land, which is completely surrounded by a body of water all the time. Not just a few months of the year, but the whole year. I told my friend, there are a very few islands in Budapest meeting this criteria.
We have to wind back the clock till the dawn of written history. By the Budapest section it means the time when the first Roman legionaries settled down next to the Danuvios river. Until this time there was no human impact on the Danube valley. No dredging, no locks, no stone embankments and no bridges.