Great steps forward can be made for improving quality of life with small human engineering waterworks and small destructions of natural flows

How to travel the fastest route from the Pacific Ocean to Tokyo, with air distance of 100 km?

Tone is the largest river in Japan, it supplies 37 million inhabitants of the Tokyo Metropolitan Area with water nowadays. Tone River was flowing towards Tokyo Bay more than 400 years ago, when engineers redirected it towards the Pacific Ocean, where the new river mouth was dislocated for 100 kilometers. It is not known what was the main reason for the diversion of the river. One hypothesis of historians, from the 19^th century, is the defense of Tokyo (then Edo) from floods. Second one is raising of low river water levels for easier navigation on river routes from the Pacific Ocean to Tokyo, which attracted more attention in the 20^th century.

We analyzed the second hypothesis for the main reason for Tone River diversion using the H08 hydrological model. We discussed what kind of knowledge we can obtain from the reconstruction of historical truths and how today's humanity can profit from the obtained knowledge. We successfully reconstructed the hydrological cycle of the Tone River, showing that the second assumption of historians about the main reason for diverting the river to raise its minimum levels is correct. The minimum levels of the Tone River increased significantly after its diversion. Because of this, the possibility of uninterrupted navigation of ships through the river waterways of the Pacific Ocean to Tokyo has been significantly increased. As a result of the increased navigability of ships, the possibilities of faster transportation of goods, cargo and commodity exchange increased the quality of social life that took place at the river ports.

This study presents the first distributed hydrological simulation confirming the claims raised by historians that the Tone River Eastward Diversion Project in Japan was conducted four centuries ago to increase low flows and subsequent travelling possibilities surrounding the Capitol Edo using inland navigation. We reconstructed six historical river maps and indirectly validated simulations with reachable ancient river ports via increased low-flow water levels.

The Tone River diversion project is used as a proof that small human engineering waterworks can greatly improve people's quality of life, without excessive destruction of natural flows. Learning from history should become a more important factor in challenging climate changes.

Our study is one of pioneering research in new discipline "History of Hydrology" (by Keith Beven), making a bridge between two disciplines. We believe that it will encourage broader scientific audience to engage in transdisciplinary hydrological and related studies by providing insights in historical engineering and scientific knowledge. There are a lot of present-day scientific efforts focused on projection, prediction and forecasting of near-future or far-future scenarios, yet historical studies are often sidelined. If only scientific community realizes that, sometimes at tipping points of climate, we can learn from past more than from future, then we believe that the historical cross-disciplinary insights will create abundant new approaches.

Keywords: Paleo-hydrological bridge; H08 global hydrological model; Tone River Eastward Diversion; 17^th century; maps reconstruction; low flows; navigable paths;