That

Geography

That (of Latin to conceal, rapid) is born in the Massif Central to 713 m from altitude on the commune of Calvinet and is thrown in the Lot with height of the commune of Bouziès (Batch), close to Saint-Cirq-Lapopie.

Departments and main cities crossed

Aveyron
Cantal: Saint-Constant
Batch: Bagnac-on-That, Figeac, Marcilhac-on-That, Espagnac-Holy-Eulalie, Cabrerets

Hydrology

That has a rain food, which explains the changes of its flow, the brutal Crue S at the time of Orage S, which can take, sometimes, a catastrophic character.

The river runs out according to a rather strong slope (210 m of uneven on 136 km of course) and its basin is rather reduced and homogeneous. Its flow thus depends much on precipitations, which are reflected quickly on the river.

That is an abundant river, like the majority of the Aquitanian rivers resulting from the Massif Central. Its flow was observed over one 37 years period (1971-2007), with Orniac, locality located not far from its confluence with the Lot. The catchment area of the river is there of 1 194 km ².

The interannual medium flow or module of the river with Orniac is of 18,6 m ³ a second.

That presents seasonal fluctuations of flow extremely marked. The high waters are held in winter and in spring and carry the average monthly flow on a level located between 21 and 37 m ³ a second, from December to May included (with an in February maximum). However as of March, a fall is already felt. The flow falls in June what leads quickly to the low waters of summer, which take place from July to September included, involving a fall of the monthly medium flow up to 3,72 m ³ in August, which remains however consistent. But the fluctuations of flow can be much more significant over shorter periods like according to the years.

With the low water level the VCN3 can fall until 0,84 m ³, in the event of dry quinquennial period, that is to say 840 liters a second, which is not very severe 'to see note).

As for the risings, as already announced they can be extremely important, characteristic shared by the majority of the affluents of the Batch and the Garonne. The QIX 2 and QIX 5 or calculated flows of biennial and quinquennial rising are worth respectively 250 and 360 m ³. The QIX 10 or calculated flow of decennial rising is worth 440 m ³ a second, while the QIX 20 is assembled to 510 m ³. Finally the QIX 50 is assembled to 600 m ³ a second (see note).

The recorded maximum daily output with Orniac during this period, was of 517 m ³ a second on December 4th, 2003, while the maximum daily output was assembled to 371 m ³ a second on December 14th, 1981. By comparing the first of these two values on a QIX scale of the river, one notes that this rising was of a vicennial nature and thus by no means exceptional.

It is interesting to compare these flows of raw That with those of two important affluents of the Seine upstream of Paris, the Marne and the Yonne both at the end of the course (ref.: and). Whereas the QIX 10 of That is assembled to 440 m ³, that of the Marne to the doors of Paris is worth 510 m ³, while that of Yonne to its outlet is of 710 m ³. Thus, the risings of relatively modest That are worth more than 80% of those of the Marne whose basin is ten times vaster, and more than 60% of the dreaded flows of rising of Yonne, whose basin is nine times wider.

On the whole, That is to it an abundant river. The Lame of water past in its catchment area is of 495 millimetres annually, which is largely higher than the overall average of France all confused basins, and even definitely higher than the average of the basin of the Lot (446 millimetres at the end of the course) however strongly supplied with important precipitations of the solid masses of the Cantal and Aubrac inter alia. The specific Débit of the river (or Qsp) posts of this fact a solid figure: 15,6 liters a second and per square kilometer of basin.

References

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