This earthcache is placed with the kind permission of CADW
A river is not a living organism but it does move and wriggle around its floodplain. All watercourses have a tendancy to meander and there are many explanations as to why. For instance a simple explanation is that the flow gets diverted from the straight by a fallen tree and once the river begins to kink, it continues to form bends of ever-increasing degree and complexity. There are more scientific explanations to do with spreading energy loss or the sediment load but for this earthcache, all we need to know is that the Tywi (or Towy in its anglicised form) has been an active river on its floodplain for thousands of years.
The movement of a river around its floodplain leaves a micro relief and this earthcache explains this micro relief and then asks you to identify features. There are potentially eight micro relief features on a floodplain and a ninth on the edge. These are:
1. The river channel itself.
2. Abandoned parts of the channel which are also sometimes known as oxbow lakes but they come in all shapes and sizes
3. Point bars- areas of deposition on the inside of bends in the channel. There is a large point bar immediately down stream of the car park.
4. Meander Bars- gentle undulations in the relief of the floodplain which are successive point bars on the inside of a bend in the river. Often visible because the vegetaion may be different in the badly draining bottoms of the undulation.
5. Flood plain scour routes- these are subsidiary channels only used in floods. There is an artificial scour route about 2 miles upstream to protect an entrance road to a property. The river kept washing away the road which was on the outside of a meander but since the digging of a scour route on the inside of the meander, the road no longer gets washed away.
6. Levees- When the river floods over the top of its banks, the water in the channel is still travelling fast but the water outside the channel is moving more slowly and deposits some of the sediment it is carrying. The deposition rate is highest just at the edge of the channel and this builds up banks into levees or "raised banks".
7. Backswamps- these are large swampy areas occupying low areas on the floodplains and often at the edge of the floodplain.
8. Sand and gravel splays- If the river breaks through a levee whilst in flood, there will often be an area of sediment deposited below the break where the water slows down.
The ninth "edge" feature are river terraces. If the floodplain drops for some reason, remnants of the former floodplain are often left behind at the edge of the floodplain. Beween the two levels there is a steep bank or terrace. These give a record of the change in the level of the floodplain.
So this earthcache requires you to visit two particular spots and tell me in an e mail- not in the log, please- , the features you have seen.
Park in the carpark at the coordinates and wander down to the lowest spot in the carpark enclosure (this may have water in it-if so take care and you may have to guestimate the depth of the water!).
1. What feature have you descended to get there and what height is it?
2. You are stood in a partly filled in, disused, river channel with a bit of a levee in front of you along the river bank. What height is the top of the river bank above the low point of the carpark enclosure?
Now climb the hill to the castle ruins (take care crossing the road). The waypoint coordinates will (with a bit of walking around) give you views over the whole floodplain.
3. How many meanders can you see downstream of the bridge? ( For the purpose of this a meander is any corner in the river channel greater than 30 degrees) And how many degrees is the greatest bend approximately?
4. What feature can you see in the inside of the first meander upstream of the bridge? (clue- they are a series of concentric rings)
Now look out across the floodplain, along the B4297. The old railway line can be seen running along the floodplain, effectively cutting off half the floodplain from the river for the past 130 years. However, one can still see oxbow lakes on either side of the road as it approaches the far side of the floodplain.
5. Beyond the oxbow lakes, what floodplain feature can you see right at the far edge of the floodplain? This may be easier to see from the road itself, but it is visible from the castle.
Please e mail me the answers to the above five questions. As soon as you have e mailed me, you may post your "found it" log. And most important of all, if you haven't previously done the trad cache on the Castle mound, don't forget to do it as well.