Abusive use of Upper Stream River was demonstrated by China in years - when the downstream residents in Thailand suffered from water shortage to flooding -- China controlled the dam water in upper stream, and downstream people had no say in how water is sustained for everyday use.
By building cascades of large dams on international rivers just before they leave its territory, China is re-engineering cross-border natural flows. Among the rivers it has targeted are the Mekong, the lifeline of Southeast Asia, and the Brahmaputra, the lifeblood for Bangladesh and northeastern India.
China has gone into overdrive to appropriate natural resources. On the most essential resource, freshwater, it is seeking to become the upstream controller by manipulating transboundary flows through dams and other structures.
Each country may have different policy/laws with respect to any water body.
The more serious problem arises when the water is shared by two regions/ two different regulations and countries -- ie US x Mexico Border Region / Transboundary issue. So for India, rivers shared by neighboring countries like Pakistan, Bangladesh have more complex issues -- as you probably know better.
Waters that provide a channel for commerce and transportation of people and goods.
Under U.S. law, bodies of water are distinguished according to their use. The distinction is particularly important in the case of so-called navigable waters, which are used for business or transportation. Jurisdiction over navigable waters belongs to the federal government rather than states or municipalities.
Construction of Dam/Check dam/ Diversion canals etc. requires environmental clearance from MOEF and state PCB's etc. EIA report to be submitted to get the clearance. Law is there for every country to protect the environment and it may slightly vary from country to country.
There is the "Convention on the Protection and Use of Transboundary Watercourses and International Lakes", maybe there are others, otherwise inter-country agreements and treaties.
For diverting the course of natural river, one must construct some structure. And for constructing that structure on the river one must take Environmental Clearance from Ministry of Environment, Forest and Climate Change (MoEFCC) in India by preparing an ESIA report that will identify the potential environmental and social issues impacted by the proposed project.
This would differ from country to country. Should the diversion affect another country, the country planning such diversion should consult the country/countries affected.
In the US it is governed under the Clean Water Act and administered by the respective states with primacy and the US Corps of Engineers for non-primacy states.
In the news today on dam in Ethiopia--https://www.middleeastmonitor.com/20180927-egyptian-minister-of-irrigation-announces-failure-of-technical-negotiations-on-grand-ethiopian-renaissance-dam/
Each country does indeed write its own laws, but maybe it also helps to know there can be big differences between countries. Some countries have no legislation, but in others the entitlement to use water is covered by property legislation. In the UK, water in river courses cannot be "owned", so is not subject to property laws. Instead it is covered by environmental legislation which requires any abstraction from the river (including a complete diversion) to be allowed by an abstraction licence, which can only be obtained in the first place from the Environment Agency. Otherwise, any interference with the flow of a river is illegal.
Diversion can impact many nations, so depending where your project is it could be governed by a water rights treaty. E,g, Jordan River impacts many countries. Ethiopia is having issues with a new dam and flow reduction--currently negotiating treaty but the dam's project manager was murdered by opposition to the dam water restrictions. Very sensitive project requiring Environmental Impact Studies on affected parties and mutual agreements.
It depends upon what country you are in. In the US, yes there are rules and regulations including wetlands, river dredging, and a host of other regulations.