Searching for carp fisheries – Castelnau lake – France

Carp fishing fanatics are constantly looking for attractive waters. Most of them set off into the unknown only then, when they manage to "track" a new fishery in conversations with fellow anglers or friends they meet by the water.
Many anglers only use maps for that, to get to the selected fishery. It's high time to change that. By analyzing the map, we can get to know the future big carp fishery relatively in detail before we set off into the unknown! Of course, not all maps are suitable for this purpose. Large overview maps, for example a map of France to scale 1:600 000, they only provide very general information about it, where rivers flow and the largest lakes are located. From a scale car map 1:200 000 we will find out much more details. However, a full and detailed analysis of the unknown fishery can only be made from a map with a scale 1:25 000. What information read from the map may be useful for a carp angler? The most interesting thing is, of course, the depth of the new fishery. It is even marked on some maps, although this information is missing in most cases. But even then the angler is able to accurately estimate the depth of a given body of water. All you need to do is take a close look at the spirit levels (lines connecting points of the same height above sea level) surrounding a given body of water. It is so, that just in the dam lakes, the shape of the shore surface looks the same (or very similar) like under water. What does it mean, I will try to explain using the example of the Lac de Castelnau reservoir, the famous carp fishery in the French Central Massif. The depth of the fishery can be easily calculated. The backwaters of the lake (where the backwater of the river ends) begins at a height 412 meters above sea level. The base of the dam wall that holds the lake is only high 373 meters.
Height difference of approx 39 meters is therefore the sought maximum depth at the dam wall! Since this lake is characterized by a variable water level, our calculations will be reasonably accurate only with the maximum damming of the water.

Fishing grounds

Our further search will concern the location of good fisheries. For this purpose, we will need not only the knowledge of the maximum depth of the body of water, but above all, an idea of ​​the shape of the surface of the falls going down to the water. Because there are no depth lines marked on the map (the so-called. izobat), as a point of reference we must use the contours of the slopes of the mountains surrounding the lake. The difference in height between the two contours marked on the map is ten meters. The closer the contours on the mountain slope are to each other, this makes the slope steeper. All clear?
In the case of our example lake, it is best seen near Lous. To the east of this town, the slope of the mountain enters the water very steeply. Further south, the contours marked on the map are further apart. So the slope of the mountain is much gentler. Evidence: the slope of the mountain has lowered from height 447 meters to the height 412 meters (height above sea level of the shoreline of the lake) on the episode approx 200 meters. So the difference in height is 35 meters. On the other hand, the terrain is getting lower 60 meters per stretch 125 meters, so you can see, that at the northern and southern shores, the slopes are very different.

Shallow shore

Further considerations will prove, that the course of bank slopes under water can be guessed on the basis of observation of the topography in the immediate vicinity of the shore. Imagine a river valley. Edge, on which the water presses and was constantly washed away, it is always steeper than the opposite shore (the inner shore of the old trough), where the water-borne material is deposited. Let's take a look at the map, to the peninsula east of Lous; at the northern shore, the contours are closer to each other, than on the south shore. In short: the shore of Lac de Castelnau at the height of Lous (the inflow of the river and the beginning of the lake basin) it is relatively shallow. The strip of deep water runs along the old river bed, with the greatest wash, so at today's south shore.

The land formation zone

Now everything will be clearer. Our interpretation of the map would be quite accurate, were it not for a factor, which we also need to take into account. During the year in the river, on the basis of which the dam lake was created, the water level is very variable, and this, in turn, determines the strength and speed of the current. What does it matter? The river's current carries sand, gravel, leaves and other sedimentary materials. The moment the flowing water flows into the lake, he starts to slow down immediately, its transport power is also decreasing. The heaviest "load", for example gravel, it settles on the bottom right at the inflow to the lake. The lighter sand is carried a little further, while the floating parts spread over the entire lake. Near the "mouth" of the river (where the backwater ends), roughly from Lous to Cabanac, sediments are constantly depositing at the bottom of the lake, it's getting shallower, in a word, it is a zone of slow land formation. We can assume, that at Lous the lake is much shallower than near the opposite shore (spirit levels, old trough), and given the sedimentation process, we can venture to say, that a great shallow has formed there. In August 1993 the year the fishing results confirmed the attractiveness of this fishery. Lac de Castelnau midline, stretching from Cabanac almost to the dam is narrow and resembles a meandering river. In some places, the width of the lake does not exceed a hundred meters. The shores are extremely steep on both sides (the contours on the map are very close to each other) and the bank falls under water look similar.

Deep stagnant water

The lake widens at the last bend. It is noteworthy, that an additional dam had to be built near Disses, to flood the entire valley. In order to know the shape of the bottom near the dam, it should be compared with the contours above the power plant (C. Electr) at the main dam. Isobats under water are arranged in exactly the same way.

Effect: There are no shallows in the immediate vicinity, the bottom drops almost vertically down to a depth of approx 39 meters. In other words – regardless of the distance, on which we will cast the bait, the line running along the slope will always run straight down into the water. Theory? So, but during my fishing trips it was confirmed to the letter. Only, what I could not determine from the map is a fact, that the whole fishery was literally littered with floating and sunken bits of trees.

The map can also be used to precisely define the type of the dam lake bottom. The bottom of the lake is only slightly changed by the flooding of the natural substrate of the area with water. The vegetation on the ground has, of course, died, but the tree trunks may still be in the water. If a forest once grew in the flooded area, we always have to take into account a large number of snags. Whether it will be a problem during fishing, we will find out during the initial sounding of the fishery.

In lowland dam lakes, for example in Lac du Der, at the bottom there is a humus layer that existed even before the lake was formed. For this reason, a layer of silt of varying thickness is deposited on the bottom and shores of such lakes over time. In mountain dam lakes, sedimentation zones are formed in the river "inflow" zone, in which the materials carried by the river are deposited.
Looking from the "beginning" of the lake basin, the bottom is gravel at first, then sandy, and only a little farther on, a layer of silt appears on it.

The bottom is mostly rocky near the dam that ends the lake, while in shallower reservoirs, rocks may also be covered with a layer of sand or silt.
On the basis of the map, unfortunately it is impossible to determine, whether it occurs in a given lake, and if so, this is what aquatic vegetation. Nevertheless, such factors, like "depth, nature of the bottom, or current ”allow us to draw far-reaching conclusions.

If the water level in the lake fluctuates by several meters – in dam lakes this is perfectly normal – many plant species do not find favorable conditions for vegetation in such water. Summary: At the beginning, it seems very difficult to precisely locate your dream fishery according to the map. However, I advise you to try! Thanks to this, you will certainly experience many unforgettable adventures, and besides, you'll be one step closer to the fish of your dreams.

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