In the semi-tropical climate of Babylonia the canals played a vitally important part for the successful prosecution of agriculture, and it was to the royal interest to see that their channels were kept in a proper state of repair and cleaned out at regular intervals. There is evidence that nearly every king of the First Dynasty of Babylon cut new canals and extended the system of irrigation and transport by water that he had inherited. The rich silt carried down by the rivers was deposited partly in the canals, especially in those sections nearer the main stream, with the result that the bed of a canal was constantly in process of being raised. Every year it was necessary to dig this deposit out and pile it upon the banks. Every year the banks rose higher and higher, until a point was reached when the labour involved in getting rid of the silt became greater than that required for cutting a new channel. Hence sections of a canal were constantly being recut alongside the old channel, and it is probable that many of the canals, the cutting of which is commemorated in the texts, were really reconstructions of older streams, the beds of which had become hopelessly silted up. At the present day the traveller in certain parts of Babylonia comes across the raised embankments of old canals extending across the plain within a short distance of each other, and their parallel course is to be explained by the process of recutting, which was put off as long as possible, but was at last necessitated by the growing height of the banks. As the bed of a canal gradually rose too, the high banks served the purpose of retaining the stream, and these were often washed away by the flood-water which came down from the hills in spring. An interesting letter has been preserved, that was written by Hammurabi's grandson, Abi-eshu', who describes a sudden rise of this sort in the level of the Irnina Canal so that it overflowed its banks.[12] At the time the king was building a palace in the city of Kâr-Irnina, which was supplied by the Irnina Canal, and every year a certain amount of work was put into the building. At the time the letter was written little more than a third of the year's work had been done, when the building-operations were stopped by flood, the canal having overflowed its banks so that the water rose right up to the town-wall.