A combination of heavy rain and melted snow caused rivers in the region to burst their banks.
More than 80 homes have been flooded in Newcastleton and 12 streets were left underwater after the storm swept through the Borders.
Rest centres were set up overnight at the Teviotdale Leisure Centre, where 22 people attended, and at the health centre in Newcastleton, where 12 people attended.
A spokesperson from Scottish Borders Council said three rivers in the Borders reached the highest on SEPA’s records and Newcastleton reportedly experienced some of the worst flooding in its history.
Hawick’s Slitrig Water, the Jed Water at Chesters and the Liddel Water at Newcastleton saw a fast rise in water levels, and the Ettrick Valley was also affected by flood water as well as Peebles.
Two people were rescued by emergency services after their car was swept off the road by flood waters, near Newcastleton. They were taken to hospital and have since been discharged.
Sepa flood duty manager Mark McLaughlin said on Saturday: “Severe flood warnings for areas in the Scottish Borders have been issued tonight as we continue to see rivers respond to the impacts of Storm Dennis.
“These warnings mean that extensive flooding is expected to properties and businesses with many roads impassable. Some evacuations have been advised.
“The severe flood warnings have been issued for Teviot and Slitrig at Hawick and Newcastleton. We advise people to stay away from flood water and to not take unnecessary risks…
“If you are affected or concerned about flooding please contact the emergency services."
Dumfries and Galloway
In Dumfries, drivers were asked to move their cars from the East car park after the Whitesands burst its banks on Saturday evening.
Although river levels are now receding, they are expected to remain high during Sunday and a flood alert remains in place.
Dumfries & Galloway VOST say there may be ongoing localised flooding, primarily of land and roads, with local disruption to travel and difficult driving conditions possible.
Annandale and Eskdale Police ask the community to expect travel disruption because of very strong winds across Dumfries and Galloway. Delays and cancellations to rail, air and ferry services as well as road closures are expected.
Police say coastal communities may be affected by spray from the sea and there is a chance of damage to buildings and power cuts.
South of the border more than thirty flood warnings were put in place for Cumbria however the county avoided the worst of Storm Dennis' havoc.
Around 12 flood warnings are still in place for the area and numerous roads are closed due to surface water.
People living in Appleby saw the River Eden burst its banks on the town's main road for the second time in a week.
Cumbria County Council is urging motorists not to drive through the flood waters and ask people to pay attention to safety barriers in place.
The Environment Agency has said the conditions in Cumbria were not as severe as Storm Ciara which hit the nation last weekend.
Windermere Ferry will not operate on Sunday due to high winds and river levels. The water is too high for passengers to safely access the ferry via the ramps.
Avanti West Coast says it's currently planning to run services as normal but that passengers should check before they travel.
Relief from Storm Dennis is still some distance away, the Met Office’s Mr Dewhurst said, adding that “heavy rain and strong winds” would continue across large parts of the UK on Sunday.
“South Wales will see a lot of rain fall before Dennis moves north later to Scotland and Northern Ireland.”
A further alert for high winds has been issued from 12:00pm on Sunday to 12:00am on Monday covering most of the country.