大家好，
最近在讀一篇文章時，讀到一句很長的句子，且有多個標點符號，
從文法上不太懂怎樣斷句，想請教大家。
原句如下：
行號
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.
之前有學過兩個逗號之間的句子是要額外多描述一些逗號之前的名詞，
所以1,2,3行加起來，可以看成1+3, 2可以先不要看變成如下：
Sediments are also dropped where a river slows on entering a lake or the sea
but will be located inland at some future date.
但因為多了第4行，請問第4行是用來描述前面的date麼? 還是我的理解有錯誤。
感謝。