River Wharfe. The River Wharfe is a river in Yorkshire, England. For much of its length it is the county boundary between West Yorkshire and North Yorkshire. Its valley is known as Wharfedale. The river source is at Beckermonds in Langstrothdale in the Yorkshire Dales National Park and flows through Kettlewell, Grassington, Bolton Abbey, Addingham, Ilkley, Burley-in-Wharfedale, Otley, Wetherby and Tadcaster. It then flows into the River Ouse near Cawood. The section of the river from its source to around Addingham is in Upper Wharfedale and has a very different character to the river downstream. The Wharfe is 65 miles long before it joins the Ouse making it the 21st longest river in Britain. It is a public navigation from the weir at Tadcaster to its junction with the Ouse near Cawood and is tidal from Ulleskelf to the Ouse. The river source is the confluence of Oughtershaw Beck and Green Field Beck in Langstrothdale near Beckermonds. It flows east and south-east taking the flows from many small streams, whose sources are the sink holes from Yockenthwaite Moor on the north bank and Horse Head Moor on the south bank. After Hubberholme, the river flows south past Buckden and Starbotton and then south-east past Kettlewell before flowing south again. Near Conistone are the gentle waterfalls of Scar Lash. Near Grassington, the river turns south-east over Linton Falls. South of Appletreewick, the river flows south-west for a short distance until it reaches Gill Beck and returns southward. To the north of Bolton Bridge, the river narrows and goes over waterfalls in an area known as The Strid. The river winds south and south-east towards Ilkley where it heads east through Otley and Collingham. It briefly flows north to Wetherby before turning south and then south-east through Tadcaster to the confluence with the River Ouse. More than 230 species of bird have been observed along the river valley including eagle-owl, red grouse, stonechat, whinchat, golden plover, pied flycatcher, redstart, wood warbler, common sandpiper, grey wagtail, dipper, tawny owl, sparrowhawk, greater spotted woodpecker, nuthatch, treecreeper and, in wetter places, snipe and woodcock, chiffchaff, willow warbler, garden warbler, and twite. The Wharfe has populations of signal crayfish and the few white-clawed crayfish remaining are at great risk. The river is also home to a colony of fine-lined pea mussels. The creature is mostly congregated in rivers in Wales and central and southern England; the presence of the mussels in the Wharfe has been noted as a considerable outpost. There are populations of rabbit, red fox, grey squirrel, otter, water voles and deer.