Where you find shallow water in streams you will sometimes find a mat of leaves with small yellow flowers amongst them, this is the opposite-leaved golden saxifrage (Chrysosplenium oppositifolium). The leaves are visible for much of the year but the flowers only from February until May. I have said that the flowers are yellow but that is not entirely true, it is the impression the plant gives. In reality the plant has no petals but it does have yellowish green sepals and bright yellow anthers.
This is quite a distinctive plant and quite unmistakable when you find it. The opposite-leaved golden saxifrage is common across Dorset in suitable habitat but the alternate-leaved golden saxifrage is absent from the county, it is a mountain species, and so confusion should not arise.