Keep it Clean
One of the main considerations when taking care of a stream in your garden is keeping it clean. It should go without saying that, in order to protect the local environment and local wildlife, you should always garden organically. It is vitally important that you do not release any nasty chemicals such as pesticides or herbicides into the waters that flow across your land.
Another thing to consider is that you should take care not to disrupt the banks of the stream too much – excess mud and silt can slow the flow of the water, impede the progress of wildlife and make it less appealing to aquatic life.
Finally, you should also take care not to send contaminants downstream in the form of seeds or sections from invasive, non-native plant species. Lysichiton americanus (skunk cabbage) for example, can spread along the banks of streams, out-competing native vegetation. Be sure to remove this plant if you find it along your stream, and make sure you are not part of perpetuating and proliferating this non-native species or other problem plant species.
Keep it Cold
Like land-living creatures, fish and other aquatic creatures living in your stream need oxygen to breathe. Cold water can hold more oxygen than warm water. For this reason, streams that are colder will have a greater diversity of creatures in them. You can help to increase biodiversity by helping to make sure your stream remains icy cold. You can do this by ensuring a good coverage of shade by planting native trees and shrubs to shield your stream from the hot sun during the summer months. Keeping a stream moving as quickly as possible will also help to keep its temperature down in the warmest part of the year.
Keep it Complex
Just as you should aim to increase and sustain diversity by ensuring there are a good range of habitats throughout the land in your garden, you should also do what you can to maintain a diverse range of aquatic environments in your stream as it flows through your garden. Try to maintain a 'riffle-pool-run' structure in the steam, or restore this structure if it has been lost. You may be able to add or remove stones and rocks from the stream bed to create a range of shallow areas, slow moving, deeper pools and runs of moderate depth and speed.