大家好，
最近在读一篇文章时，读到一句很长的句子，且有多个标点符号，
从文法上不太懂怎样断句，想请教大家。
原句如下：
行号
1 Sediments are also dropped where a river slows on entering a lake or the sea,
2 the deposited sediments are on a lake floor or the seafloor at first,
3 but will be located inland at some future date,
4 when the sea level falls or the land rises;
5 such beds are sometimes thousands of meters thick.
之前有学过两个逗号之间的句子是要额外多描述一些逗号之前的名词，
所以1,2,3行加起来，可以看成1+3, 2可以先不要看变成如下：
Sediments are also dropped where a river slows on entering a lake or the sea
but will be located inland at some future date.
但因为多了第4行，请问第4行是用来描述前面的date么? 还是我的理解有错误。
感谢。