Isostasy | A1 Sample answer
Explain how rivers adjust to a change in base level with reference to examples you have studied (2013 Q1C)
Isostatic uplift is when land rises above sea level because of tectonic activity. This is usually due to a large weight being removed from the land e.g. when an ice cap melts. Eustatic changes are when the sea level drops. This is due to water being locked away somewhere e.g., in a glacier. When it melts, the sea level rises again.
The River Nore in Kilkenny has experienced a change in base level. This can be seen since there are knickpoints along it, roughly 150-200m above sea level. A knickpoint can be seen when the sea level drops and a river rejuvenates because the river starts vertically eroding once more. Rejuvenation is the term used to describe a river that starts eroding like a youthful river even when it is in its old age stage. A knickpoint is a spot where the newly eroded river profile meets the old river profile. Sometimes you will find a waterfall here.
River terraces are also found where rejuvenation has occurred. This is when a river forms a new floodplain that is lower down than the last. The remnants of the old floodplain are left as steps and are called terraces. If there are terraces on both sides of the river, they are called paired terraces. Terraces can be formed multiple times if the river rejuvenates more than once, appearing as a series of steps down to the river.
You may also find incised meanders on a rejuvenated river. If a river starts eroding vertically as well as horizontally, it will create incised meanders. There are two types of these meanders. Entrenched meanders are known for their symmetrical cross section and form if the sides of the river valley resist the erosion. They can also be created if the incision is rapid. Ingrown meanders are caused by slow uplift of land and this allows the river to move laterally across its floodplain. Incised meanders can be seen on the River Nore at St. Mullins, Co. Kilkenny.
1 Isostasy | Sample answer